For Aristotle, the nutritive capacity of plants is in analogy with the perceptive and intellective capacity of human beings.
Outside of Aristotle’s own arguments, we might considerate that the last capacity is in need to be understood from the process of nutrition.
Before Aristotle, Plato speaks about knowledge as feeding: Surely, I said, knowledge is the food of the soul; and we must take care, my friend, that the Sophist does not deceive us when he praises what he sells, like the dealers wholesale or retail who sell the food of the body; for they praise indiscriminately all their goods, without knowing what are really beneficial or hurtful (Protagoras).
The knowledge as food is presented as something that can be beneficial or hurtful. It is so only when is taught. Otherwise, we could imagine that knowledge would feed the man like as plants do, that is, by assimilating the food in the process of growing from a seed that is not fed yet.
In the case of men, the seed will be constituted by their soul. However, soul is a vague notion and the soul conceived as human ‘self’ involves that the seed and its feeding would bear the mark of individuality.
The progression in knowledge as a simile of plants’ capacity of nutrition would support a vague and individual self-understanding. But there is still feasible to think that the initial vagueness and individuality will grow by a continual denial of their nature. A cognitive vagueness cannot develop in a greater state of vagueness, since, as we learn from Monet, a great image of vagueness becomes a forceful whole. Nonetheless, the same do Plato’s interrogative inquiries. In the same manner, the individuality strengthens when is supported by general values.
Contrary to such ‘vegetative’ nutrition of soul, the knowledge as a food sold by others has the virtue of being firm when is presented as beneficial and the virtue of being already put in general terms.
The virtues of such knowledge often surpass its lacks. Instead of a laborious waiting for discovering if the food sold is beneficial or hurtful, it dissipates the discomfort of feeling ourselves in the uncertainty of a fundamental vagueness and of a fragile individuality.