Themistius illustrates this obscure passage, p. 89. The definitions of [Greek: eu)thei=a grammê/, keklasme/nê grammê/, peripherê\s grammê/], must each of them contain the definition of [Greek: grammê/ (= mê=kos a)plate/s)], since it is in the Category [Greek: Poso/n (poso\n mê=kos a)plate/s)]. But the derivative properties of the circle ([Greek: peripherê\s grammê/]) are deduced from the definition of a circle, and belong to it in the first instance _quâ_ [Greek: peripherê\s grammê/], in a secondary way _quâ_ [Greek: grammê/].] [Footnote 40: Analyt. Post. II. xiii. p. 96, b. 25-p. 97, a. 6.] Some contemporaries of Aristotle, and among them Speusippus, maintained that it was impossible either to define, or to divide logically, unless you knew all particulars without exception. You cannot (they said) know any one thing, except by knowing its differences from all other things; which would imply that you knew also all these other things.[41] To these reasoners Aristotle replies: It is not necessary to know _all_ the differences of every thing; you know a thing as soon as you know its essence, with the properties _per se_ which are derivative therefrom. There are many differences not belonging to the essence, but distinguishing from each other two things having the same essence: you may know the thing, without knowing these accidental differences.[42] When you divide a genus into two species, distinguished by one proximate specific difference, such that there cannot be any thing that does not fall under one or other of these _membra condividentia_, and when you have traced the subject investigated under one or other of these members, you can always follow this road until no lower specific difference can be found, and you have then the final essence and definition of the subject; even though you may not know how many _other_ subjects each of the two members may include.[43] Thus does Aristotle reply to Speusippus, showing that it is not necessary, for the definition of one thing, that you should know _all_ other things. His reply, as in many other cases, is founded on the distinction between the Essential and the Accidental. [Footnote 41: Ibid. p. 97, a. 6-10; Themistius, p. 92. Aristotle does not here expressly name Speusippus, but simply says [Greek: phasi/ tines]. It is Themistius who names Speusippus; and one of the Scholiasts refers to Eudemus as having expressly indicated Speusippus (Schol. p. 248, a. 24, Br.).] [Footnote 42: Analyt. Post. II. xiii. p. 97, a. 12: [Greek: pollai\ ga\r diaphorai\ u(pa/rchousi toi=s au)toi=s tô=| ei)/dei, a)ll' ou) kat' ou)si/an ou)de\ kath' au(ta/.]] [Footnote 43: Ibid. a. 18-22: [Greek: phanero\n ga\r o(/ti a)\n ou(/tô badi/zôn e)/lthê| ei)s tau=ta ô(=n mêke/ti e)sti\ diaphora/, e(/xei to\n lo/gon tê=s ou)si/as.]]