Twentieth century philosophers have distinguished between descriptive metaphysics and revisionist metaphysics. Aristotle’s metaphysics is clearly an attempt to describe, analyze, and justify the common beliefs about humanity and the world, not an attempt to persuade people to revise their prephilosophical views of the world in some radical fashion. Unless the revisionist metaphysics of Aristotle’s contemporaries is understood, however, it is impossible to understand Aristotle’s own accomplishment.
Previous philosophers, such as Heraclitus, argued that the only source of knowledge is that which is observed through one of the five senses, and since the testimony of the five senses reveals a continually changing world, it follows that absolutely nothing remains the same. A rock or a mountain may at first seem fairly stable, but close examination reveals that they, too, are continually being diminished by the winds and the rains. As Heraclitus said, it is impossible to step into the same river twice. Rocks and mountains may not change as quickly, but they change no less surely.
To be told that rivers, rocks, and mountains are continually changing appears to be relatively innocuous. Yet the logic of Heraclitus’s argument makes it impossible to stop there. If the only source of knowledge is through the senses, then absolutely everything must be in a continual state of flux. A person who robs a bank, for example, can never be caught because whoever is charged with the crime is necessarily a different person than the one who actually committed it. Heraclitus’s philosophical conclusions are clearly in radical opposition to the commonsense view of the world.
Other philosophers, such as Parmenides, argued for the exact opposite conclusion, namely, that all change is illusory. While Heraclitus appealed to empirical data, Parmenides appealed to reason. Consider everything that really exists in the entire universe precisely as it is at this particular instance, he believed. Whatever that “everything” is, it is by definition the Real, and anything else must therefore be unreal. Now if the Real were to change, it would become something that it is not, that is, it would become unreal. Yet the unreal does not exist. Thus, for anything to change is for it to become nonexistent. All change must therefore be unreal.
The radical opposition of Parmenides’ philosophical conclusions are obvious from the start. What is not so obvious is exactly where his reasoning is mistaken. While the common people will be able to continue their daily tasks without ever addressing either Heraclitus or Parmenides’ arguments, it would be inconsistent for Aristotle to insist that first principles must be dialectically justified and then simply ignore these revisionist arguments. Commonsense assumptions must be justified.
The three assumptions that Aristotle seeks to justify are, first, that things exist; second, that some things move and change; and finally, that the things in this universe that exist, move, and change are not totally unintelligible. The common element of all three beliefs is the notion of a “thing.” What is a thing? Aristotle says that things have being (existence) and that a metaphysician’s task is to make clear exactly what being is. In fact, he often defines the subject matter of metaphysics as the study of all things insofar as they exist.
Compare this definition with the definition of other disciplines. The subject matter of physics, says Aristotle, is things insofar as they are moving or changing objects. The subject matter of biology is things insofar as they are alive. The subject matter of ethics is things insofar as they are able to make rational choices between competing goods. One notices how the various subject matters of different disciplines constitute a hierarchical series from the particular to the general. Thus, a single person can be studied on at least three different levels. First, her or she can be studied by the moral philosopher as a “thing” capable of making rational choices. At a more general level, he or she can be studied by the biologist as a “thing” that is alive. At an even more general level, her or she can be studied by the physicist as a “thing” that moves.
The crucial metaphysical question for Aristotle thus becomes the following: Is there any more general level at which one can study things than at the level of the physicist? Aristotle thinks that there is, namely, at the level at which things are studied simply insofar as they exist. This way of defining the different disciplines ensures that no important questions are begged. In particular, it leaves open the question of whether anything exists apart from space and time. One of the important conclusions in the Metaphysics is that such a being, the unmoved mover or God, does exist. Yet before addressing such interesting and difficult theological questions, Aristotle wisely directs his attention to the more mundane, but almost as difficult, question, What is a thing?
Aristotle begins by cataloging the ordinary sorts of things that exist in this universe. There is this particular rock, that particular tree, and his friend Theaetetus. The point of any catalog is to organize...
(The entire section is 2154 words.)