"Neither of these writers has given us an adequate rule," he said. "For if Cato intended, as he doubtless did, that we should add to or subtract from what he prescribes in proportion as our farm is of greater or less extent than that he describes, he should have excluded the overseer and the housekeeper from his enumeration. If you cultivate less than two hundred and forty jugera of olives you cannot get along with less than one overseer, while if you cultivate twice or more as much land you will not require two or three overseers. It is the number of labourers and teamsters only which must be added to or diminished in proportion to the size of the farm: and this applies only if the land is all of the same character, for if part of it is of a kind which cannot be ploughed, as for example very rocky, or on a steep hillside, there is that much less necessity for teams and teamsters. I pass over the fact that Cato's example of a farm of two hundred and forty jugera is neither a fair nor a comparable unit.[78] The true unit for comparison of farms is a centuria, which contains two hundred jugera, but if one deducts forty jugera, or one-sixth, from Cato's two hundred and forty jugera, I do not see how in applying this rule one can deduct also one-sixth of his thirteen slaves; or, even if we leave out the overseer and the housekeeper, how one can deduct one-sixth of eleven slaves. Again, Cato says that one should have fifteen slaves for one hundred jugera of vineyard, but suppose one had a _centuria_ half in vines and half in olives, then, according to Cato's rule, one would require two overseers and two housekeepers, which is absurd. Wherefore it is necessary to find another measure than Cato's for determining the number of slaves, and I myself think better of Saserna's rule, which is that for each jugerum it suffices to provide four days work of one hand. Yet, if this was a good rule on Saserna's farm in Gaul, it might not apply on a mountain farm in Liguria. In fine you will best determine what number of slaves and what other equipment you will require if you diligently consider three things, that is to say, what kind of farms are there in your neighbourhood, how large are they, and how many hands are engaged in cultivating them, and you should add to or subtract from that number in proportion as you take up more or less work. For nature gave us two schools of agriculture, which are experience and imitation. The most ancient farmers established many principles by experiment and their descendants for the most part have simply imitated them. We should do both these things: imitate others and on our own account make experiments, following always some principle, not chance:[79] thus we might work our trees deeper or not so deep as others do to see what the effect would be. It was with such intelligent curiosity that some farmers first cultivated their vines a second and a third time, and deferred grafting the figs from spring to summer."