Once, then, our thoughts were not rational. That is, all our thoughts once were, as many of our thoughts still are, merely subjective events, not apprehensions of objective truth. Those which had a cause external to ourselves at all were (like our pains) responses to stimuli. Now natural selection could operate only by eliminating responses that were biologically hurtful and multiplying those which tended to survival. But it is not conceivable that any improvement of responses could ever turn them into acts of insight, or even remotely tend to do so. The relation between response and stimulus is utterly different from that between knowledge and the truth known. Our physical vision is a far more useful response to light than that of the cruder organisms which have only a photo-sensitive spot. But neither this improvement nor any possible improvements we can suppose could bring it an inch nearer to being a knowledge of light. It is admittedly something without which we could not have had that knowledge. But the knowledge is achieved by experiments and inferences from them, not by refinement of the response. It is not men with specially good eyes who know about light, but men who have studied the relevant sciences. In the same way our psychological responses to our environment — our curiosities, aversions, delights, expectations — could be indefinitely improved (from the biological point of view) without becoming anything more than responses. Such perfection of the non-rational responses, far from amounting to their conversion into valid inferences, might be conceived as a different method of achieving survival — an alternative to reason. A conditioning which secured that we never felt delight except in the useful nor aversion save from the dangerous, and that the degrees of both were exquisitely proportional to the degree of real utility or danger in the object, might serve us as well as reason or in some circumstances better.