text: You will say, I suppose: "What is the purpose and meaning of this preamble?" I shall not keep
  you in the dark; I desire, if possible, to say the word essentia to you and obtain a favourable hearing. If I cannot do this, I shall risk it even though it put you out of humour. I have Cicero, as authority for the use of this word, and I regard him as a powerful authority. If you desire testimony of a later date, I shall cite Fabianus, careful of speech, cultivated, and so polished in style that he will suit even our nice tastes. For what can we do, my dear Lucilius? How otherwise can we find a word for that which the Greeks call οὐσία, something that is indispensable, something that is the natural substratum of everything? I beg you accordingly to allow me to use this word essentia. I shall nevertheless take pains to exercise the privilege, which you have granted me, with as sparing a hand as possible; perhaps I shall be content with the mere right. Yet what good will your indulgence do me, if, lo and behold, I can in no wise express in Latin the meaning of the word which gave me the opportunity to rail at the poverty of our language? And you will condemn our narrow Roman limits even more, when you find out that there is a word of one syllable which I cannot translate. "What is this?" you ask. It is the word ὄν. You think me lacking in facility; you believe that the word is ready to hand, that it might be translated by quod est. I notice, however, a great difference; you are forcing me to render a noun by a verb. But if I must do so, I shall render it by quod est. There are six ways in which Plato expresses this idea, according to a friend of ours, a man of great learning, who mentioned the fact to-day. And I shall explain all of them to you, if I may first point out that there is something called genus and something called species.