 As for the proof for the existence of minima naturalia itself, it begins by repeating that division is spoken of in two ways: physical division—which brings about an actual division, severance or fragmentation of the magnitude—and conceptual division—which involves a mere mental division or positing that leaves the magnitude intact. Again the argument is specifically about physical divisibility. The argument runs thus: The smaller the quantity of a given substance the more apt it is to be acted upon by surrounding bodies. So, for example, all things being equal, it takes longer for a body of water to cool down a ton of molten iron than for that same body of water to cool down an ounce of molten iron, and similarly, a blast furnace is able to heat the same ounce more quickly than the ton. So again the smaller the physical divisions of a given substance are, the more disposed that reduced quantity of the substance becomes to the primary qualities of the surrounding bodies. Below a certain limit, the ratio between the strength of the primary qualities of the surrounding bodies and those of the body being divided is such that the qualities of the surrounding bodies overcome those of the divided body. At that moment, the divided substance’s underlying qualitative disposition becomes altered so as no longer to be suitable to its elemental form, and the matter receives a new substantial form. So, for example, imagine a cup of water that is surrounded by hot, dry summer air. Now imagine half that amount of water, and then keep taking halves. At some point the amount of water is so small that the water simply evaporates as it were instantaneously, or, as medieval natural philosophers would have it, the form of water in that minuscule physical quantity is immediately replaced with the form of air. In short, for these thinkers the elements, as well as more complex mixtures, have natural minima beyond which they cannot be divided and still be capable of retaining their species form, for the primary qualities of the surrounding bodies so alter that body that it is no longer suitable for sustaining its initial form. There must be, then, the argument concludes, minima naturalia.