1. _The System of Numbers_. Thus the original and simple proposition of the Pythagorean philosophy is, according to Aristotle (Metaph. I. 5), “that number is the reality of things, and the constitution of the whole universe in its determinations is an harmonious system of numbers and of their relations.” In what sense is this statement to be taken? The fundamental determination of number is its being a measure; if we say that everything is quantitatively or qualitatively determined, the size and measure is only one aspect or characteristic which is present in everything, but the meaning here is that number itself is the essence and the substance of things, and not alone their form. What first strikes us as surprising is the boldness of such language, which at once sets aside everything which to the ordinary idea is real and true, doing away with sensuous existence and making it to be the creation of thought. Existence is expressed as something which is not sensuous, and thus what to the senses and to old ideas is altogether foreign, is raised into and expressed as substance and as true Being. But at the same time the necessity is shown for making number to be likewise Notion, to manifest it as the activity of its unity with Being, for to us number does not seem to be in immediate unity with the Notion. Now although this principle appears to us to be fanciful and wild, we find in it that number is not merely something sensuous, therefore it brings determination with it, universal distinctions and antitheses. The ancients had a very good knowledge of these. Aristotle (Metaph. I. 6) says of Plato: “He maintained that the mathematical elements in things are found outside of what is merely sensuous, and of ideas, being between both; it differs from what is sensuous in that it is eternal and unchangeable, and from ideas, in that it possesses multiplicity, and hence each can resemble and be similar to another, while each idea is for itself one alone.” That is, number can be repeated; thus it is not sensuous, and still not yet thought. In the life of Pythagoras, this is further said by Malchus (46, 47): “Pythagoras propounded philosophy in this wise in order to loose thought from its fetters. Without thought nothing true can be discerned or known; thought hears and sees everything in itself, the rest is lame and blind. To obtain his end, Pythagoras makes use of mathematics, since this stands midway between what is sensuous and thought, as a kind of preliminary to what is in and for itself.” Malchus quotes further (48, 53) a passage from an early writer, Moderatus: “Because the Pythagoreans could not clearly express the absolute and the first principles through thought, they made use of numbers, of mathematics, because in this form determinations could be easily expressed.” For instance, similarity could be expressed as one, dissimilarity as two. “This mode of teaching through the use of numbers, whilst it was the