Stagira, 384–322 BC

Aristotle

To know a thing is to know why it is so and that it could not be otherwise. Aristotle built the first theory of science on that standard, and rested the whole structure on induction.

The view

Aristotle studied under Plato for twenty years and then rejected his teacher's separate world of Forms. The forms of things are in the things, he held, and we come to know them through the senses. He opens the Metaphysics with the claim that all human beings by nature desire to know.

His account of scientific knowledge is in the Posterior Analytics. We have knowledge in the full sense, epistēmē, when we know the cause on which a fact depends and know that the fact could not be otherwise. Such knowledge is set out as demonstration: a chain of deductions from premises that are true, primary, immediate, and explanatory of the conclusion. Geometry was his model. A finished science would look like a geometry textbook.

That raises an obvious question. Demonstrations start from first principles. If those had to be demonstrated too, the chain would never begin. So how are first principles known?

The argument

Aristotle answers in the last chapter of the work. First principles are neither inborn nor proved. They are reached by epagōgē, usually translated "induction", in four stages. Animals have perception. In some, perceptions persist as memory. Many memories of the same thing make up a single experience. And out of experience the universal comes to rest in the soul.

It is like a rout in battle: one man makes a stand, then another, until the original line is restored. Posterior Analytics II.19, paraphrased

The state of mind that then grasps the principle he calls nous, intellect or insight. It is more certain than demonstration itself, since demonstration depends on it.

Where he meets Deutsch

Aristotle puts explanation at the centre. To know is to know why, and he draws the line between knowing that something is so and knowing the reason. A list of true facts is not yet science. Deutsch says the same against every view that treats theories as mere instruments for prediction: the purpose of science is to explain.

He was also a working investigator, of animals above all, who took evidence seriously and expected reality to be intelligible. Both attitudes are close to Deutsch's realism.

Where they part

The ladder is the problem. No number of remembered cases contains a universal, and Hume showed that no valid argument leads from the cases to the law. Aristotle half sees the gap and bridges it with nous, a faculty that simply sees the principle to be true. For Deutsch there is no such faculty. The universal is a guess. It may be prompted by experience, but it is not extracted from it, and it never becomes certain.

The picture of finished science fails for the same reason. Aristotle's ideal is a completed body of necessary truths deduced from secure foundations. Deutsch's is an unending sequence of explanations, each replacing the last. The history of Aristotle's own physics makes the case. It was treated as settled for the better part of two thousand years, and when it fell it was replaced by a better explanation, not corrected by further observation.

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Further reading

• Aristotle, Posterior Analytics, translated with commentary by Jonathan Barnes, 2nd edn (Oxford, 1994). Book I.2 for demonstration; II.19 for first principles.
• Jonathan Barnes, Aristotle: A Very Short Introduction (Oxford, 2000).
• Aristotle's Logic, Stanford Encyclopedia of Philosophy, for demonstration and induction.