Chapter 3: Of the five modes of attaining truth: (1) of demonstrative science of things invariable.

Let us describe these virtues then, starting afresh from the beginning.

Let us assume that the modes in which the mind arrives at truth, either in the way of affirmation or negation, are five in number, viz. art, science, prudence, wisdom, reason; for conception and opinion may be erroneous.

What science is we may learn from the following considerations (for we want a precise account, and must not content ourselves with metaphors). We all suppose that what we know with scientific knowledge is invariable; but of that which is variable we cannot say, so soon as it is out of sight, whether it is in existence or not. The object of science, then, is necessary. Therefore it is eternal: for whatever is of its own nature necessary is eternal: and what is eternal neither begins nor ceases to be.

Further, it is held that all science can be taught, and that what can be known in the way of science can be learnt. But all teaching starts from something already known, as we have explained in the Analytics; for it proceeds either by induction or by syllogism. Now, it is induction that leads the learner up to universal principles, while syllogism starts from these. There are principles, then, from which syllogism starts, which are not arrived at by syllogism, and which, therefore, must be arrived at by induction.

Science, then, may be defined as a habit or formed faculty of demonstration, with all the further qualifications which are enumerated in the Analytics. It is necessary to add this, because it is only when the principles of our knowledge are accepted and known to us in a particular way, that we can properly be said to have scientific knowledge; for unless these principles are better known to us than the conclusions based upon them, our knowledge will be merely accidental.

This, then, may be taken as our account of science.