In the physical constitution of an organized being,....

“ In the physical constitution of an organized being, that is, a being adapted suitably to the purposes of life, we assume it as a fundamental principle that no organ for any purpose will be found but what is also the fittest and best adapted for that purpose. Now in a being which has reason and a will, if the proper object of nature were its conservation, its welfare, in a word, its happiness, then nature would have hit upon a very bad arrangement in selecting the reason of the creature to carry out this purpose. For all the actions which the creature has to perform with a view to this purpose, and the whole rule of its conduct, would be far more surely prescribed to it by instinct, and that end would have been attained thereby much more certainly that it ever can be by reason. Should reason have been communicated to this favored creature over and above, it must only have served it to contemplate the happy constitution of its nature, to admire it, to congratulate itself thereon, and to feel thankful for it to the beneficent cause, but not that it should subject its desires to that weak and delusive guidance, and meddle bunglingly with the purpose of nature. In a word, nature would have taken care that reason should not break forth into practical exercise, nor have the presumption, with its weak insight, to think out for itself the plan of happiness and the means of attaining it. Nature would not only have taken on herself the choice of the ends but also of the means, and with wise foresight would have entrusted both to instinct. ”

Immanuel Kant

From : Fundamental Principles of the Metaphysic of Morals