温伯格《解释世界》第三章 运动与哲学

柏拉图之后希腊探索自然世界的风格发生了转变，不再诗意化而是更加好辩。这种变化在亚里士多德的工作中尤为突出。亚里士多德于公元前384年出生于马其顿的斯塔基拉，他不是雅典或爱奥尼亚人。公元前367年他搬到雅典在柏拉图创建的学校—柏拉图学院学习。公元前347年柏拉图去世后亚里士多德离开雅典，在莱斯博斯的爱琴岛和海滨城市阿索斯生活了一段时间。公元前343年亚里士多德被马其顿腓力二世召回去辅导他的儿子亚历山大，即后来的亚历山大大帝。 腓力的军队于公元前338年在克罗尼亚战役中击败雅典和底比斯之后，马其顿开始统治希腊。公元前336年腓力去世后亚里士多德回到雅典，在那里他创建了自己的学校—吕刻俄斯。这是雅典四所伟大学校之一，其他三所为柏拉图学院，伊壁鸠鲁花园和斯多葛画廊。吕刻俄斯学校一直持续了几个世纪，可能一直到公元前86年苏拉统治下的雅典被罗马军队洗劫后才关闭。柏拉图学院持续更久，以不同形式一直持续到公元529年，比目前欧洲所有大学的历史都悠久。 亚里士多德流传下来的著作主要是他在吕刻俄斯授课时的讲稿。他们涉及的科目惊人：天文学，动物学，梦，形而上学，逻辑，伦理，辩论，政治，审美学，以及通常被翻译为“物理”的学科。据一位现代译者介绍，亚里士多德的希腊文“精炼，简洁，跳跃，他的论证精简，思维缜密，”与柏拉图的诗意风格完全不同。我承认我发现亚里士多德的文字常常很沉闷，而柏拉图的文字不会。但是虽然亚里士多德常常会犯错，但他不糊涂，然而柏拉图有时会糊涂。 柏拉图和亚里士多德都是现实主义者，但两者的现实主义体现在不同意义之上。柏拉图是中世纪意义上的现实主义者：他相信抽象观念下的现实，特别是事物的理念形式。松树真实的是它的理念形式，而不是单个松树，那只是这种形式的不完美反映。只有形式不变，这也是巴门尼德和芝诺所坚持的。亚里士多德是现代意义上的现实主义者：对他来说，虽然种类极为有趣，但是单个个体才是真实的，像单个松树，而不是柏拉图的形式。 亚里士多德小心的用推理而不是用启示来证实他的结论。我们认同古典学者汉金森的观点：“我们不要忽略亚里士多德是他那个时代的巨人的事实—对于那个时代来说他极富洞察力，头脑敏锐，而且博学。”但是在发现现代科学的过程中亚里士多德思想中的一部分原则必须被摒弃。 首先亚里士多德的著作中充满了目的论：事情之所以以某种形式存在是由于他们所服务的目的。在《物理学》中我们读到：“自然就是结果或目的。如果某一事物历经持续变化而终结，那么最后阶段实际上就是它的最终目的。” 这种对目的论的重视对非常关注生物学的亚里士多德来说再自然不过。亚里士多德曾在阿索斯和莱斯博斯学习海洋生物，他的父亲尼克马库斯曾经是马其顿宫廷御医。几位比我更了解生物学的朋友告诉我亚里士多德对动物的描述令人钦佩。任何研究动物心脏或胃等器官的学者（比如写《动物之构造》的亚里士多德）倾向目的论都很正常--他自然会疑问为什么动物要生长这些器官。 事实上一直到19世纪达尔文和华莱士的进化学说之后自然学家才开始认识到虽然器官有不同作用，但是它们的演化没有任何目的性。它们之所以是现在这样是经历数百万年不定向遗传变异的结果。当然在达尔文之前物理学家早已学会研究物质和力而不去问它们的目的。 亚里士多德早期对动物学的关注可能也使他非常重视分类学—即将事物按类型分开。其中的一些分类我们至今还在应用，比如亚里士多德政体分类：君主政体，贵族政体以及非民主但宪制政体。但是多数分类没有意义。我可以想象亚里士多德如何对水果进行分类：所有水果可以分为三类—苹果，橙子以及非苹果和橙子的所有其他水果。 亚里士多德有一种分类遍布于他的研究中，这种分类后来成为科学发展的障碍。他坚持自然与人为的区别。在《物理学》卷二开头他写道：“存在的事物中一些是自然存在的，一些是由于其他原因，只有自然存在的才值得关注。”可能正是由于这种对自然和人为的区分才致使亚里士多德和他的追随者对实验没有兴趣。如果真正有趣的是自然现象，那创建人为情形有什么好处？ 亚里士多德并没有忽略对自然的观察。从看到闪电与听到雷声的时差，或从看到远方三层浆座战船划桨到听到它们发出的声音，他确定声音是以一定速度传播。我们也会看到他通过观察得知地球的形状以及彩虹的原由。但这全都是对自然现象的偶尔观察，不是为了实验的目的而去建立人为环境。 对自然与人为的区分极大地影响了亚里士多德对科学史上一个非常重要问题—落体的认识。亚里士多德讲授实体之所以下落是因为元素土的自然位置朝下，向着宇宙的中心。而火上升是由于火的自然位置在天上。地球是近球形，其中心位于宇宙的中心，因为这样才能使土趋向中心。同样如果让物体自然下落，其下落速度会正比于落体重量。亚里士多德在《天论》中介绍：“一定重量的物体在一定时间内运动一定的距离。重的物体运动同样距离用时要短，时间与重量成反比。例如如果一个物体是另一个物体的两倍重，这个物体运动同样距离耗时是另一个的一半。”我们不能责怪亚里士多德完全忽视对落体的观察。虽然他并不知道原由，空气或其他围绕落体的介质阻力确会使落体速度最终接近常值—称为终速。随着落体重量增加，终速增加（见技术说明6）。可能对亚里士多德更为重要的是落体速度随着重量增加而增加的观察与他的物体下落是由于物体的自然位置向着宇宙中心的思想相符。 对亚里士多德来说，空气或其他介质的存在对理解运动至关重要。他认为如果不存在阻力的话物体运动速度会达到无限大，这过于荒谬，因此上他否认虚空存在的可能。在《物理学》中他主张“让我们来解释根本不存在有些人所坚信的虚空。”但事实上只是落体的终速与阻力成反比。在完全没有阻力的情况下，终速确实是无穷大，但是那样的话落体永远不会达到终速。 亚里士多德在同一章提出了一个更为复杂的论点--在虚空里运动没有什么相对性:“虚空中物体一定处于静止状态；因为在虚空中不存在物体可以相对运动多少的地方。虚空就是这样，内部不具有差别。”但是这个论点应该只针对无限虚空，否则的话虚空中的运动可以相当于虚空外的任何物体。 由于亚里士多德只熟知存在阻力条件下的运动，因而他相信任何运动都有因（注）（亚里士多德指出四种因：质料因，形式因，动力因，及最终因，最终因是目的因—是变化的目的）。（注：希腊字“kineson”常被翻译为”运动“， 事实上其具有广义，泛指任何变化。亚里士多德对运动原因的分类不是只适用于位置的变化，而是适用于任何变化。希腊字”fora”专指位置的变化，通常被翻译为“运移”）。因一定是由其他因所导致，一直这样下去，但是不能无限追溯下去。我们在《物理学》里可以看到，“因为任何物体的运动必然是有推动者在推着它运动。让我们设想一个物体在运移，该物体受到一个自身也在运移的物体的推动，推动者又受到其他物体的推动，一直下去；然而这个序列不会无限继续，必然存在着一个第一推动者。”这个第一推动者的教条后来成为基督教和伊斯兰教相信上帝存在的论据。但是我们后面会看到，上帝不能创造虚空的结论在中世纪给亚里士多德信仰伊斯兰教和基督教的追随者带来了麻烦。 亚里士多德并不因实体不总是向其自然位置运动的事实而困扰。手里握着的石头不会下落，对亚里士多德来说这只是说明了人为因数对自然次序的干扰。但他确实会对向上扔出去的石头的运动问题发愁，石头先会上升一段时间，远离地球，即使石头这时已经离开了手。他是这样解释的（其实等于没有解释），石头之所以会继续向上运行一段时间是因为空气赋予的动力。在《天论》第三卷中他解释道：“力传递运动到物体是通过首先在空气中的向上作用。这就是为什么受推动物体即使推动力停止作用后仍然会持续运动的原因”。我们会看到该教义在古代和中世纪常常会被拿来讨论或被摒弃。 亚里士多德有关落体的写作风格至少在他的物理里表现的非常典型—他基于假想的第一原理通过非数学推理进行阐明，第一原理本身也只是基于偶尔地对自然的观察，他完全没有试图去测试这些原理。 我并不是说亚里士多德的追随者和继承者把亚里士多德的哲学看作另一套科学。在古代和中世纪还没有不同于哲学的科学概念。探索自然属于哲学范畴。一直到19世纪，当德国大学要为从事艺术和科学学者颁发一个与神学，法律和医学博士学位相当的学位时，他们引入了“哲学博士”学位。早期当人们对比哲学和其他自然学科时，对比的不是科学，而是数学。 哲学史上亚里士多德的影响力无与伦比。在第九章我们会看到一些阿拉伯哲学家对他推崇备至。甚至连阿维罗伊对他也一样遵从。在第十章我们会介绍13世纪亚里士多德如何成为基督教欧洲颇具影响力的人物。这期间托马斯·阿奎那将他的思想与基督教进行融合。在中世纪盛期人们直接称呼亚里士多德为“哲学家”，称呼阿维罗伊为“注释家”。阿奎那之后学习亚里士多德成为大学教育的核心内容。 在乔叟的《坎特伯雷故事》开场白中介绍了一个牛津学者： 也有一位牛津学者 他宁可在床头放置 二十卷亚里士多德哲学书 或红或黑装订 却不讲究衣着，也不拉提琴，弹弦乐。 当然现在情况完全不同。在发现科学进程中把科学与现今称之为哲学的学科分开是至关重要一步。现在虽然科学哲学很活跃，也颇为有趣，但其对科学研究基本毫无影响。 在第十章介绍的起源于十四世纪的早期科学革命很大程度上是对亚里士多德学说的否定。最近一些亚里士多德追随者开展了一场反向革命。极具影响力的历史学家托马斯·库恩描述了他如何从轻蔑到敬仰亚里士多德的转变： 特别是有关运动方面，他的描述在我看来充满严重错误。无论在逻辑方面还是从观察角度，我觉得这些结论都是不太可能的，毕竟亚里士多德曾经是备受敬仰的古代逻辑编纂者。即使他去世两百年后，他在逻辑方面的成就仍然发挥着重要作用。就像欧几里德在几何方面的成就一样。。。。。。。为什么转向研究运动和力学后他的天才能力会系统地丧失哪？同样地，为什么他的物理方面的理论即使在他离世几百年一直备受关注哪？。。。。。。突然间我脑海中的各个片段贯穿在一起，我完全理清了头绪。我惊讶的下巴都掉了下来，因为我一下明白亚里士多德事实上是一位杰出的物理学家，但是是那种我做梦也想不到的物理学家。。。。。。我突然找到了读懂亚里士多德的方法。 在我们一起获得帕多瓦大学荣誉学位的时候我听到他的上述言论。后来我让他做出解释，他回应说：“通过我自己阅读（亚里士多德物理学内容）所改变的是我对这些物理成就的理解，而非我的评价。”对此回答我不甚明白：“事实上是一位杰出的物理学家”在我看来就是评价。 关于亚里士多德对实验不感兴趣这点，历史学家大卫·林德伯格做出如此评述：“因此不应当将亚里士多德的科学实践解读为他自身的愚昧或瑕疵（他没有能意识到明显的可改进空间），而是应解读为那是与他所理解的世界相匹配的一种方法，这种方法非常适合于他所感兴趣的问题。” 至于如何评价亚里士多德的成就这样宏大的问题，林德伯格补充说：“用亚里士多德对现代科学的预见程度来评价亚里士多德的科学成就显然不公平而且毫无意义，就好像他的研究目的是为了解决我们现代的问题，而不是他当时面临的问题。”在该书的再版中又说：“正确评价哲学体系或科学理论不是去评价其对现代思想的预见程度，而是其在处理那个时代的哲学和科学问题时取得多大成就。” 这种观点我难以接受。对科学（我把哲学留给别人讨论）至关重要的不是去解决一个人自己时代的一些流行科学问题，而是去认识世界。在此进程中，人们探求答案，研究有助于找到答案的问题。科学进展很大程度上是去发现哪些问题适合研究。 当然人们也应该去理解科学发现的时代背景。除此之外，历史学家的任务取决于他或她想要实现什么。如果历史学家的目的只是为了重现过去，去了解“过去究竟是什么样的”，那么用现代标准去评价过去没有用处。但是如果想了解科学从它的过去到现代的进展过程，那这种评价就是必不可少的。 这个进步是客观的，不是时尚的演化。人们有可能去怀疑牛顿比亚里士多德更了解运动吗？会怀疑我们比牛顿认识更深吗？去问什么运动是自然的，或这种或那种物理现象的意义是什么这样的问题是不会有用的。 认为亚里士多德愚蠢的结论是不公平的，这点我赞同林德伯格的观点。这里我用现代标准评判过去的目的是为了说明我们学会如何认识自然有多么不易，即使如亚里士多德这般才智过人。现代科学实践对没有见识过的人来说完全不是显而易见的。 在亚历山大于公元前323年去世后亚里士多德离开了雅典，之后不久在公元前322年离世。迈克尔·马修斯说他的去世“标准着人类历史上一段最为辉煌智识时代的落幕。”这确实是古典时代的结束。但是正如我们即将看到的，这也标志着科学史上更为辉煌时代的开端—希腊化时代。

After Plato, the Greeks’ speculations about nature took a turn toward a style that was less poetic and more argumentative. This change appears above all in the work of Aristotle. Neither a native Athenian nor an Ionian, Aristotle was born in 384 BC at Stagira in Macedon. He moved to Athens in 367 BC to study at the school founded by Plato, the Academy. After the death of Plato in 347 BC, Aristotle left Athens and lived for a while on the Aegean island of Lesbos and at the coastal town of Assos. In 343 BC Aristotle was called back to Macedon by Philip II to tutor his son Alexander, later Alexander the Great. Macedon came to dominate the Greek world after Philip’s army defeated Athens and Thebes at the battle of Chaeronea in 338 BC. After Philip’s death in 336 BC Aristotle returned to Athens, where he founded his own school, the Lyceum. This was one of the four great schools of Athens, the others being Plato’s Academy, the Garden of Epicurus, and the Colonnade (or Stoa) of the Stoics. The Lyceum continued for centuries, probably until it was closed in the sack of Athens by Roman soldiers under Sulla in 86 BC. It was outlasted, though, by Plato’s Academy, which continued in one form or another until AD 529, enduring longer than any European university has lasted so far. The works of Aristotle that survive appear to be chiefly notes for his lectures at the Lyceum. They treat an amazing variety of subjects: astronomy, zoology, dreams, metaphysics, logic, ethics, rhetoric, politics, aesthetics, and what is usually translated as “physics.” According to one modern translator,1 Aristotle’s Greek is “terse, compact, abrupt, his arguments condensed, his thought dense,” very unlike the poetic style of Plato. I confess that I find Aristotle frequently tedious, in a way that Plato is not, but although often wrong Aristotle is not silly, in the way that Plato sometimes is. Plato and Aristotle were both realists, but in quite different senses. Plato was a realist in the medieval sense of the word: he believed in the reality of abstract ideas, in particular of ideal forms of things. It is the ideal form of a pine tree that is real, not the individual pine trees that only imperfectly realize this form. It is the forms that are changeless, in the way demanded by Parmenides and Zeno. Aristotle was a realist in a common modern sense: for him, though categories were deeply interesting, it was individual things, like individual pine trees, that were real, not Plato’s forms. Aristotle was careful to use reason rather than inspiration to justify his conclusions. We can agree with the classical scholar R. J. Hankinson that “we must not lose sight of the fact that Aristotle was a man of his time—and for that time he was extraordinarily perspicacious, acute, and advanced.”2 Nevertheless, there were principles running all through Aristotle’s thought that had to be unlearned in the discovery of modern science. For one thing, Aristotle’s work was suffused with teleology: things are what they are because of the purpose they serve. In Physics,3 we read, “But the nature is the end or that for the sake of which. For if a thing undergoes a continuous change toward some end, that last stage is actually that for the sake of which.” This emphasis on teleology was natural for someone like Aristotle, who was much concerned with biology. At Assos and Lesbos Aristotle had studied marine biology, and his father, Nicomachus, had been a physician at the court of Macedon. Friends who know more about biology than I do tell me that Aristotle’s writing on animals is admirable. Teleology is natural for anyone who, like Aristotle in Parts of Animals, studies the heart or stomach of an animal—he can hardly help asking what purpose it serves. Indeed, not until the work of Darwin and Wallace in the nineteenth century did naturalists came to understand that although bodily organs serve various purposes, there is no purpose underlying their evolution. They are what they are because they have been naturally selected over millions of years of undirected inheritable variations. And of course, long before Darwin, physicists had learned to study matter and force without asking about the purpose they serve. Aristotle’s early concern with zoology may also have inspired his strong emphasis on taxonomy, on sorting things out in categories. We still use some of this, for instance the Aristotelian classification of governments into monarchies, aristocracies, and not democracies but constitutional governments. But much of it seems pointless. I can imagine how Aristotle might have classified fruits: All fruits come in three varieties—there are apples, and oranges, and fruits that are neither apples nor oranges. One of Aristotle’s classifications was pervasive in his work, and became an obstacle for the future of science. He insisted on the distinction between the natural and the artificial. He begins Book II of Physics4 with “Of things that exist, some exist by nature, some from other causes.” It was only the natural that was worthy of his attention. Perhaps it was this distinction between the natural and the artificial that kept Aristotle and his followers from being interested in experimentation. What is the good of creating an artificial situation when what are really interesting are natural phenomena? It is not that Aristotle neglected the observation of natural phenomena. From the delay between seeing lightning and hearing thunder, or seeing oars on a distant trireme striking the water and hearing the sound they make, he concluded that sound travels at a finite speed.5 We will see that he also made good use of observation in reaching conclusions about the shape of the Earth and about the cause of rainbows. But this was all casual observation of natural phenomena, not the creation of artificial circumstances for the purpose of experimentation. The distinction between the natural and artificial played a large role in Aristotle’s thought about a problem of great importance in the history of science—the motion of falling bodies. Aristotle taught that solid bodies fall down because the natural place of the element earth is downward, toward the center of the cosmos, and sparks fly upward because the natural place of fire is in the heavens. The Earth is nearly a sphere, with its center at the center of the cosmos, because this allows the greatest proportion of earth to approach that center. Also, allowed to fall naturally, a falling body has a speed proportional to its weight. As we read in On the Heavens,6 according to Aristotle, “A given weight moves a given distance in a given time; a weight which is as great and more moves the same distance in a less time, the times being in inverse proportion to the weights. For instance, if one weight is twice another, it will take half as long over a given movement.” Aristotle can’t be accused of entirely ignoring the observation of falling bodies. Though he did not know the reason, the resistance of air or any other medium surrounding a falling body has the effect that the speed eventually approaches a constant value, the terminal velocity, which does increase with the falling body’s weight. (See Technical Note 6.) Probably more important to Aristotle, the observation that the speed of a falling body increases with its weight fitted in well with his notion that the body falls because the natural place of its material is toward the center of the world. For Aristotle, the presence of air or some other medium was essential in understanding motion. He thought that without any resistance, bodies would move at infinite speed, an absurdity that led him to deny the possibility of empty space. In Physics, he argues, “Let us explain that there is no void existing separately, as some maintain.”7 But in fact it is only the terminal velocity of a falling body that is inversely proportional to the resistance. The terminal velocity would indeed be infinite in the absence of all resistance, but in that case a falling body would never reach terminal velocity. In the same chapter Aristotle gives a more sophisticated argument, that in a void there would be nothing to which motion could be relative: “in the void things must be at rest; for there is no place to which things can move more or less than to another; since the void in so far as it is void admits no difference.”8 But this is an argument against only an infinite void; otherwise motion in a void can be relative to whatever is outside the void. Because Aristotle was acquainted with motion only in the presence of resistance, he believed that all motion has a cause.* (Aristotle distinguished four kinds of cause: material, formal, efficient, and final, of which the final cause is teleological—it is the purpose of the change.) That cause must itself be caused by something else, and so on, but the sequence of causes cannot go on forever. We read in Physics,9 “Since everything that is in motion must be moved by something, let us take the case in which a thing is in locomotion and is moved by something that is itself in motion, and that again is moved by something else that is in motion, and that by something else, and so on continually; then the series cannot go on to infinity, but there must be some first mover.” The doctrine of a first mover later provided Christianity and Islam with an argument for the existence of God. But as we will see, in the Middle Ages the conclusion that God could not make a void raised troubles for followers of Aristotle in both Islam and Christianity. Aristotle was not bothered by the fact that bodies do not always move toward their natural place. A stone held in the hand does not fall, but for Aristotle this just showed the effect of artificial interference with the natural order. But he was seriously worried over the fact that a stone thrown upward continues for a while to rise, away from the Earth, even after it has left the hand. His explanation, really no explanation, was that the stone continues upward for a while because of the motion given to it by the air. In Book III of On the Heavens, he explains that “the force transmits the movement to the body by first, as it were, tying it up in the air. That is why a body moved by constraint continues to move even when that which gave it the impulse ceases to accompany it.”10 As we will see, this notion was frequently discussed and rejected in ancient and medieval times. Aristotle’s writing on falling bodies is typical at least of his physics—elaborate though non- mathematical reasoning based on assumed first principles, which are themselves based on only the most casual observation of nature, with no effort to test them. I don’t mean to say that Aristotle’s philosophy was seen by his followers and successors as an alternative to science. There was no conception in the ancient or medieval world of science as something distinct from philosophy. Thinking about the natural world was philosophy. As late as the nineteenth century, when German universities instituted a doctoral degree for scholars of the arts and sciences to give them equal status with doctors of theology, law, and medicine, they invented the title “doctor of philosophy.” When philosophy had earlier been compared with some other way of thinking about nature, it was contrasted not with science, but with mathematics. No one in the history of philosophy has been as influential as Aristotle. As we will see in Chapter 9, he was greatly admired by some Arab philosophers, even slavishly so by Averroes. Chapter 10 tells how Aristotle became influential in Christian Europe in the 1200s, when his thought was reconciled with Christianity by Thomas Aquinas. In the high Middle Ages Aristotle was known simply as “The Philosopher,” and Averroes as “The Commentator.” After Aquinas the study of Aristotle became the center of university education. In the Prologue to Chaucer’s Canterbury Tales, we are introduced to an Oxford scholar:

A Clerk there was of Oxenford also . . . For he would rather have at his bed’s head Twenty books, clad in black or red, Of Aristotle, and his philosophy, Than robes rich, or fiddle, or gay psaltery.

Of course, things are different now. It was essential in the discovery of science to separate science from what is now called philosophy. There is active and interesting work on the philosophy of science, but it has very little effect on scientific research. The precocious scientific revolution that began in the fourteenth century and is described in Chapter 10 was largely a revolt against Aristotelianism. In recent years students of Aristotle have mounted something of a counterrevolution. The very influential historian Thomas Kuhn described how he was converted from disparagement to admiration of Aristotle:11

About motion, in particular, his writings seemed to me full of egregious errors, both of logic and of observation. These conclusions were, I felt, unlikely. Aristotle, after all, had been the much- admired codifier of ancient logic. For almost two millennia after his death, his work played the same role in logic that Euclid’s played in geometry. . . . How could his characteristic talent have deserted him so systematically when he turned to the study of motion and mechanics? Equally, why had his writings in physics been taken so seriously for so many centuries after his death? . . . Suddenly the fragments in my head sorted themselves out in a new way, and fell in place together. My jaw dropped with surprise, for all at once Aristotle seemed a very good physicist indeed, but of a sort I’d never dreamed possible. . . . I had suddenly found the way to read Aristotelian texts.

I heard Kuhn make these remarks when we both received honorary degrees from the University of Padua, and later asked him to explain. He replied, “What was altered by my own first reading of [Aristotle’s writings on physics] was my understanding, not my evaluation, of what they achieved.” I didn’t understand this: “a very good physicist indeed” seemed to me like an evaluation. Regarding Aristotle’s lack of interest in experiment: the historian David Lindberg12 remarked, “Aristotle’s scientific practice is not to be explained, therefore, as a result of stupidity or deficiency on his part—failure to perceive an obvious procedural improvement—but as a method compatible with the world as he perceived it and well suited to the questions that interest him.” On the larger issue of how to judge Aristotle’s success, Lindberg added, “It would be unfair and pointless to judge Aristotle’s success by the degree to which he anticipated modern science (as though his goal was to answer our questions, rather than his own).” And in a second edition of the same work:13 “The proper measure of a philosophical system or a scientific theory is not the degree to which it anticipated modern thought, but its degree of success in treating the philosophical and scientific problems of its own day.” I don’t buy it. What is important in science (I leave philosophy to others) is not the solution of some popular scientific problems of one’s own day, but understanding the world. In the course of this work, one finds out what sort of explanations are possible, and what sort of problems can lead to those explanations. The progress of science has been largely a matter of discovering what questions should be asked. Of course, one has to try to understand the historical context of scientific discoveries. Beyond that, the task of a historian depends on what he or she is trying to accomplish. If the historian’s aim is only to re-create the past, to understand “how it actually was,” then it may not be helpful to judge a past scientist’s success by modern standards. But this sort of judgment is indispensable if what one wants is to understand how science progressed from its past to its present. This progress has been something objective, not just an evolution of fashion. Is it possible to doubt that Newton understood more about motion than Aristotle, or that we understand more than Newton? It never was fruitful to ask what motions are natural, or what is the purpose of this or that physical phenomenon. I agree with Lindberg that it would be unfair to conclude that Aristotle was stupid. My purpose here in judging the past by the standards of the present is to come to an understanding of how difficult it was for even very intelligent persons like Aristotle to learn how to learn about nature. Nothing about the practice of modern science is obvious to someone who has never seen it done. Aristotle left Athens at the death of Alexander in 323 BC, and died shortly afterward, in 322 BC. According to Michael Matthews,14 this was “a death that signaled the twilight of one of the brightest intellectual periods in human history.” It was indeed the end of the Classical era, but as we shall see, it was also the dawn of an age far brighter scientifically: the era of the Hellenistic.