On the Tilling of the Earth by Noah (11-15)

And if, besides, there are any trees which produce no fruit at all, neither good nor bad, the husbandman will cut them down too, but still he will not suffer them to be completely destroyed, but he will apply them to some appropriate use, making them into stakes and fixing them as pales all round his homestead, or using them as a fence for a city to serve instead of a wall.

For Moses says, "Every tree which bringeth not forth fruit good to eat thou shalt cut down; and thou shalt make it into stakes against the city which shall make war upon them." [*] And these trees are likened to those powers developed in words alone, which have nothing in them but mere speculation,

among which we must class medical science, when unconnected with practise, by which it is natural that such persons may be cured, and also the oratorical and hireling [*]

At all events, men say, that the ancients compared the principles of philosophy, as being threefold, to a field; likening natural philosophy to trees and plants, and moral philosophy to fruits, for the sake of which the plants are planted; and logical philosophy to the hedge or fence:

for as the wall, which is erected around, is the guardian of the plants and of the fruit which are in the field, keeping off all those who wish to do them injury and to destroy them, in the same manner, the logical part of philosophy is the strongest possible sort of protection to the other two parts, the moral and the natural philosophy;