There is thus a large body of a priori or intuitively necessary truths. As a rule, these are truths of comparison only, and in the first instance they express relations between merely mental terms. Nature, however, acts as if some of her realities were identical with these mental terms. So far as she does this, we can make a priori propositions concerning natural fact. The aim of both science and philosophy is to make the identifiable terms more numerous. So far it has proved easier to identify nature's things with mental terms of the mechanical than with mental terms of the sentimental order.
The widest postulate of rationality is that the world is rationally intelligible throughout, after the pattern of some ideal system. The whole war of the philosophies is over that point of faith. Some say they can see their way already to the rationality; others that it is hopeless in any other but the mechanical way. To some the very fact that there is a world at all seems irrational. Nonentity would be a more natural thing than existence, for these minds. One philosopher at least says that the relatedness of things to each other is irrational anyhow, and that a world of relations can never be made intelligible.[561]
With this I may be assumed to have completed the programme which I announced at the beginning of the chapter, so far as the theoretic part of our organic mental structure goes. It can be due neither to our own nor to our ancestors' experience. I now pass to those practical parts of our organic mental structure. Things are a little different here; and our conclusion, though it lies in the same direction, can be by no means as confidently expressed.
To be as short and simple as possible, I will take the case of instincts, and, supposing the reader to be familiar with [Chapter XXIV], I will plunge in medias res.
THE ORIGIN OF INSTINCTS.
Instincts must have been either
1) Each specially created in complete form, or
2) Gradually evolved.
As the first alternative is nowadays obsolete, I proceed directly to the second. The two most prominent suggestions as to the way in which instincts may have been evolved are associated with the names of Lamarck and Darwin.
Lamarck's statement is that animals have wants, and contract, to satisfy them, habits which transform themselves gradually into so many propensities which they can neither resist nor change. These propensities, once acquired, propagate themselves by way of transmission to the young, so that they come to exist in new individuals, anteriorly to all exercise. Thus are the same emotions, the same habits, the same instincts, perpetuated without variation from one generation to another, so long as the outward conditions of existence remain the same.[562] Mr. Lewes calls this the theory of 'lapsed intelligence.' Mr. Spencer's words are clearer than Lamarck's, so that I will quote from him:[563]