humans. According to Aristotle, contemplation is the only type of happy activity which it would not be ridiculous to imagine the gods having. In Aristotle's psychology and biology, the intellect is the soul (see also eudaimonia). First cause In Book VIII of his Physics, Aristotle examines the notions of change or motion, and attempts to show by a challenging argument, that the mere supposition of a 'before' and an 'after', requires a first principle. He argues that in the beginning, if the cosmos had come to be, its first motion would lack an antecedent state; and, as Parmenides said, "nothing comes from nothing". The cosmological argument, later attributed to Aristotle, thereby draws the conclusion that God exists. However, if the cosmos had a beginning, Aristotle argued, it would require an efficient first cause, a notion that Aristotle took to demonstrate a critical flaw. The purpose of Aristotle's cosmological argument, that at least one eternal unmoved mover must exist, is to support everyday change. In Aristotle's estimation, an explanation without the temporal actuality and potentiality of an infinite locomotive chain is required for an eternal cosmos with neither beginning nor end: an unmoved eternal substance for whom the Primum Mobile turns diurnally and whereby all terrestrial cycles are driven: day and night, the seasons of the year, the transformation of the elements, and the nature of plants and animals. Substance and change Aristotle begins by describing substance, of which he says there are three types: the sensible, which is subdivided into the perishable, which belongs to physics, and the eternal, which belongs to "another science". He notes that sensible substance is changeable and that there are several types of change, including quality and quantity, generation and destruction, increase and diminution, alteration, and motion. Change occurs when one given state becomes something contrary to it: that is to say, what exists potentially comes to exist actually (see potentiality and actuality). Therefore, "a thing [can come to be], incidentally, out of that which is not, [and] also all things come to be out of that which is, but is potentially, and is not actually." That by which something is changed is the mover, that which is changed is the matter, and that into which it is changed is the form. Substance is necessarily composed of different elements. The proof for this is that there are things which are different from each other and