The Development of Aristotle's Thought
In the Categories, general objects exist. In addition to individual men, man exists. Man is a general object. Man, however, is a secondary substance, not a primary substance. Aristotle, in this way, takes a significant step away from the ontology Plato presents in the Timaeus. General objects exist, but they are not substances without qualification.
In the Metaphysics, Aristotle takes a much more significant step away from Platonism. In the Metaphysics, general objects do not exist. Man and other such general objects are not only not secondary substances, they do not exist. Natural bodies fall into kinds. Socrates is a man, but the natural kind man itself does not exist.
Aristotle has become a nominalist about universals. What is said in predicating a term such as 'man' is general. This is said of all the particulars.
Aristotle's New Solution to an Old Problem
Aristotle thus rejects a crucial inference that informs the Platonic picture. For Aristotle, general facts do not require the existence of general objects.
For Aristotle, the specification of the form of a natural body of a given kind is exactly the same for every natural body of the kind. There are general facts, but general facts are not objects. Plato worried that if general facts about the natural bodies were not ground in the existence of general objects that were themselves immune from change, there would only be the various subjective and different viewpoints of human beings. Aristotle takes this worry seriously, but he offers a fundamentally different solution than Plato. Aristotle supposes that what it is to be the species of a given natural body is not subject to change, even if there are no general objects, because the forms of natural bodies, although in matter, are separate in account. To ensure that the account of what the body is in species is not subject to change, Aristotle does not separate forms from matter and thereby make their existence consist in what is said of each natural body of a given species. For Aristotle, the separateness in account of the forms of natural bodies is separateness enough.