Knowledge is measurement, and all measure implies a minimum in each kind of being. Were it possible to subdivide anything _ad infinitum_, the half would be potentially equal to the whole, and measurement frustrated. There must be a limit to division, an ultimate part, which itself has no parts, and which is the _substance_ of the composition into which it enters, the composition on the other hand being an "_accident_" of this minimum. As it is primarily a condition of measurement, the minimum differs in the different spheres of measure or knowledge to which the category of quantity applies. In magnitudes of one or two dimensions it is the point, in bodies the atom, in numbers the monad or unity. Thus number is accident of the monad, monad is the essence of number, as composition is accident of the atom, atom is essence of the composite. Again, the "sensible minimum" must be far greater than the natural or real minimum, for in so far as minimum is qualified by sensible, it is implied that the minimum is not absolutely such, but is a composite. The minimum of taste, touch, etc., must possess certain qualities, by which it has relation to sense, and these can derive only from some form of composition. In their primary form the minima of nature must be without difference; therefore that some are sensible, others not, must be due to some addition in the former.[393] Thus each species of existence, as light, moisture, vital force,[394] has its own minimum, and the minimum is relative in this sense also, that there are different kinds of existence not resolvable one into another: the absolute minimum would be God, who is also the absolute maximum. The relative minimum, accordingly, is determined either by the thought and design of the observer, or by the species of existence to which the subject belongs; nature has set limits, both lower and upper, within which the individual of any species must stay, or cease to belong to that species. Accordingly, what _one_ regards as great and composite, _another_ may take as first and minimum: the unit of one science may be analysed in another into further elements. "Pythagoras in his philosophy started with the monad and numbers; Plato with atoms, lines and surfaces; Empedocles with the four elements; the physicians with the four humours, and so on; but the Pythagorean monad is prior to the placed monad (the atom), Plato's matter of bodies to the qualified bodies of Empedocles, the four simple bodies of Empedocles to the four first combinations of these, the four humours. So to the universe the whole solar system, the sun and all its planets, may be a simple unit."[395]