On the Allegories of the Sacred Laws (1.56-1.60)

And God caused to rise out of the earth every tree which is pleasant to the sight and good for food, and the tree of life he raised in the middle of the Paradise, and also the tree of the knowledge of good and evil. He here gives a sketch of the trees of virtue which he plants in the soul. And these are the particular virtues, and the energies in accordance with them, and the good and successful actions, and the things which by the philosophers are called fitting;

these are the plants of the Paradise. Nevertheless, he describes the characteristics of these same trees, showing that that which is desirable to be beheld is likewise most excellent to be enjoyed. For of the arts some are theoretical and not practical, such as geometry and astronomy. Some, again, are practical and not theoretical, such as the art of the architect, of the smith, and all those which are called mechanical arts. But virtue is both theoretical and practical; for it takes in theory, since the road which leads to it is philosophy in three of its parts—the reasoning, and the moral, and the physical part. It also includes action; for virtue is art conversant about the whole of life; and in life all actions are exhibited.

Still, although it takes in both theory and practise, nevertheless it is most excellent in each particular. For the theory of virtue is thoroughly excellent, and its practise and observation is a worthy object to contend for. On which account Moses says that the tree was pleasant to the sight, which is a symbol of theoretical excellence; and likewise good for food, which is a token of useful and practical good.

But the tree of life is that most general virtue which some people call goodness; from which the particular virtues are derived, and of which they are composed. And it is on this account that it is placed in the centre of the Paradise; having the most comprehensive place of all, in order that, like a king, it may be guarded by the trees on each side of it. But some say that it is the heart that is meant by the tree of life; since that is the cause of life, and since that has its position in the middle of the body, as being, according to them, the dominant part of the body. But these men ought to be made aware that they are expounding a doctrine which has more reference to medical than to natural science. But we, as has been said

And of this tree Moses expressly says, that it is placed in the middle of the paradise; but as to the other tree, that namely of the knowledge of good and evil, he has not specified whether it is within or outside of the Paradise; but after he has used the following expression, and the tree of the knowledge of good and evil, he says no more, not mentioning where it is placed, in order that any one who is uninitiated in the principles of natural philosophy, may not be made to marvel at his knowledge.