Aristotle — The Metaphysics

What is known to us as metaphysics is what Aristotle called "first philosophy." Metaphysics involves a study of the universal principles of being, the abstract qualities of existence itself. 

One point that he dwells on is the law of contradictions, which essentially asserts that something cannot both be and not be at the same time. In particular, he is concerned with the relativism and even nihilism that would result from a metaphysics that allowed contradictions. 

He draws the idea of substance, of which there are three kinds: changeable and perishable, changeable and eternal, and immutable. If all substances are perishable, then ultimate destruction of everything is inevitable. But Aristotle asserts two imperishable entities: motion and time.  He asserts that if time were created, then there must have been no time before the creation, but the very concept of "before" necessitates the concept of time. On the other hand, as he argued in his works of natural philosophy, the only continuous motion must be circular. Thus he returns to the idea of the Unmoved Mover, for only such a being could generate eternal circular motion. The Unmoved Mover is the ultimate cause of the universe, and it is pure actuality, containing no matter since it is the very cause of itself. In order for the Mover to be unmoved itself, it must move in a non-physical way, by inspiring desire.

In his book XII of Metaphysics  he states that 'if the universe is of the nature of a whole, substance is its first part; and if it coheres merely by virtue of serial succession, on this view also substance is first, and is succeeded by quality, and then by quantity. At the same time these latter are not even being in the full sense, but are qualities and movements of it,—or else even the not-white and the not-straight would be being; at least we say even these are, e.g. 'there is a not-white'. Sensible substance is changeable. Now if change proceeds from opposites or from intermediates, and not from all opposites (for the voice is not-white, (but it does not therefore change to white)), but from the contrary, there must be something underlying which changes into the contrary state; for the contraries do not change. Further, something persists, but the contrary does not persist; there is, then, some third thing besides the contraries, viz. the matter. Now since changes are of four kinds—either in respect of the 'what' or of the quality or of the quantity or of the place, and change in respect of 'thisness' is simple generation and destruction, and change in quantity is increase and diminution, and change in respect of an affection is alteration, and change of place is motion, changes will be from given states into those contrary to them in these several respects. The matter, then, which changes must be capable of both states. And since that which 'is' has two senses, we must say that everything changes from that which is potentially to that which is actually, e.g. from potentially white to actually white, and similarly in the case of increase and diminution. Therefore not only can a thing come to be, incidentally, out of that which is not, but also all things come to be out of that which is, but is potentially, and is not actually.'

One might raise the question from what sort of non-being generation proceeds; for 'non-being' has three senses. 'If, then, one form of non-being exists potentially, still it is not by virtue of a potentiality for any and every thing, but different things come from different things; nor is it satisfactory to say that 'all things were together'; for they differ in their matter, since otherwise why did an infinity of things come to be, and not one thing? For 'reason' is one, so that if matter also were one, that must have come to be in actuality which the matter was in potency. The causes and the principles, then, are three, two being the pair of contraries of which one is definition and form and the other is privation, and the third being the matter.'

Further, if the causes of substances are the causes of all things, yet different things have different causes and elements, as was said; 'the causes of things that are not in the same class, e.g. of colours and sounds, of substances and quantities, are different except in an analogical sense; and those of things in the same species are different, not in species, but in the sense that the causes of different individuals are different, your matter and form and moving cause being different from mine, while in their universal definition they are the same. And if we inquire what are the principles or elements of substances and relations and qualities—whether they are the same or different—clearly when the names of the causes are used in several senses the causes of each are the same, but when the senses are distinguished the causes are not the same but different, except that in the following senses the causes of all are the same. They are (1) the same or analogous in this sense, that matter, form, privation, and the moving cause are common to all things; and (2) the causes of substances may be treated as causes of all things in this sense, that when substances are removed all things are removed; further, (3) that which is first in respect of complete reality is the cause of all things. But in another sense there are different first causes, viz. all the contraries which are neither generic nor ambiguous terms; and, further, the matters of different things are different. We have stated, then, what are the principles of sensible things and how many they are, and in what sense they are the same and in what sense different.'

Aristotle gives the Mover the name of God, but this figure is unlike most standard conceptions of a divine being. Though Aristotle asserts that it is a living creature and represents the pinnacle of goodness, it also has no interest in the world and no recognition of man, for it exists in a completely transcendent and abstract state. The activity of God–if it can be called such–is simply knowledge, and this knowledge is purely a knowledge of itself, because an abstracted being is above sense and experience and can know only what is best. He asserts of all the ancient philosophy that God is the ultimate cause of everything in the world, but it also remains completely detached.

Airistotle describes the nature of wisdom: it begins with sense perceptions, which must be translated into scientific expertise. Such knowledge requires the understanding of both facts and causes, and wisdom comes only with an understanding of the universal principles and primary causes built on this science. Aristotle's work in metaphysics is therefore motivated by this desire for wisdom, which requires the pursuit of knowledge for its own sake.

In the first section of his Metaphisics, he affirms that 'all men by nature desire to know. An indication of this is the delight we take in our senses; for even apart from their usefulness they are loved for themselves; and above all others the sense of sight. For not only with a view to action, but even when we are not going to do anything, we prefer seeing (one might say) to everything else. The reason is that this, most of all the senses, makes us know and brings to light many differences between things.'

References:

The Methaphisics of Aristotle Books, translated by W. D. Ross
Sparknotes of Aristole's Methaphisics