 What makes an organism qualify as a natural purpose is that it is “both cause and effect of itself” (§64, 371). Kant gives a preliminary explanation of this idea at §64 by calling attention to three respects in which an organism, such as a tree, stands in a causal relation to its own existence. First, in producing offspring which resemble it, a tree “produces itself as far as its species is concerned”, so that the species of the tree maintains itself in existence. Second, a tree preserves itself as an individual by taking in nourishment from outside and converting it into the kind of organic substance of which it, itself, is made. Third, and most important, the various parts of a tree mutually maintain one another in existence and hence maintain the whole tree in existence. For example, while the leaves are produced by the tree as a whole and depend on it for their growth and maintenance, they are in turn necessary for the growth and maintenance of the other parts of the tree, for example the trunk and roots. Kant also mentions a number of further phenomena illustrating the way in which an organism is “cause and effect” of itself, in particular the capacity of certain organisms to regenerate missing parts, and more generally the capacity of organisms to repair damage to themselves.