(2) _The law of contradiction_: 'Nothing can both be and not be.' (3) _The law of excluded middle_: 'Everything must either be or not be.' These three laws are samples of self-evident logical principles, but are not really more fundamental or more self-evident than various other similar principles: for instance, the one we considered just now, which states that what follows from a true premiss is true. The name 'laws of thought' is also misleading, for what is important is not the fact that we think in accordance with these laws, but the fact that things behave in accordance with them; in other words, the fact that when we think in accordance with them we think _truly_. But this is a large question, to which we must return at a later stage. In addition to the logical principles which enable us to prove from a given premiss that something is _certainly_ true, there are other logical principles which enable us to prove, from a given premiss, that there is a greater or less probability that something is true. An example of such principles--perhaps the most important example is the inductive principle, which we considered in the preceding chapter. One of the great historic controversies in philosophy is the controversy between the two schools called respectively 'empiricists' and 'rationalists'. The empiricists--who are best represented by the British philosophers, Locke, Berkeley, and Hume--maintained that all our knowledge is derived from experience; the rationalists--who are represented by the Continental philosophers of the seventeenth century, especially Descartes and Leibniz--maintained that, in addition to what we know by experience, there are certain 'innate ideas' and 'innate principles', which we know independently of experience. It has now become possible to decide with some confidence as to the truth or falsehood of these opposing schools. It must be admitted, for the reasons already stated, that logical principles are known to us, and cannot be themselves proved by experience, since all proof presupposes them. In this, therefore, which was the most important point of the controversy, the rationalists were in the right.