Full Idea
Regarding the nature of living beings in this world, reason must assume as a necessary principle that no organ, no faculty, nothing superfluous, or disproportionate to its use, hence nothing purposeless, is to be met with.
Gist of Idea
Reason must assume as necessary that everything in a living organism has a proportionate purpose
Source
Immanuel Kant (Critique of Pure Reason [1781], B425)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.454
A Reaction
Extraordinary to treat this as an a priori truth! In fact Darwin seems to have discovered that most organs have a purpose, but sometimes they have become redundant, and certainly they can be disproportionate. Did Kant really need that massive intellect?