Faced with questions like what the world is “made of”, modern people have generally assumed that it must be some kind of “stuff”. The usual presumed answer is some sort of matter-stuff, or less commonly some sort of mind-stuff.
Plato and Aristotle already suggested a radical alternative to this way of thinking that takes the accent away from “stuff” altogether. Aristotle especially developed a rich account of how we think about these kinds of things, by looking at how we express these kinds of questions, and what we are implicitly trying to get at when we ask them.
Etymologically, eidos — the Greek word we translate as “form” — seems to begin from a notion of visual appearance, with an emphasis on shape. It acquired a more abstract sense related to geometrical figure. Plato attributed great significance to the practice of geometry as an especially clear and perspicuous kind of reasoning, but he also recognized a broader kind of reasoning associated with a dialectic of question and answer, which comes into play especially where questions of value are concerned. From a point of view of ethical practice and human life generally, questions about what something really is and why this rather than that are more important than what things are made of (see What and Why). Already with Plato, “form” came to be inseparable from meaning.
Aristotle’s classic discussion of substance (ousia) in the Metaphysics starts from the idea of a substrate in which properties inhere. This most superficial level later inspired the Greek grammarians to articulate the notion of a grammatical subject of predicates. In what I think is the single greatest example of ancient dialectic, Aristotle gradually steps back from the simple notion of a substrate. Substance becomes “what it was finally to have been” a thing, at the end of the day so to speak. But then this is further interrogated to disclose the level of actuality, or what is effectively operative in a process. But it turns out that actuality is not complete in itself. What is effectively operative does not form a self-contained whole that explains everything. A fuller understanding must take into account potentiality, which leads to a transition away from simple actuality to a larger perspective of processes and degrees of actualization, in which nothing is simply given just as it is. Aristotle was especially concerned with the forms of living things, which have this character.
The more interesting senses of form for Plato and Aristotle do not refer to something that could be simply given. In line with this but in a more speculative mode, Plotinus suggested that every form somehow in a way “contains” all other forms. The poetic truth in this is that the articulation of one form depends on the articulation of other forms, and while everything in some sense coheres, we have no unconditional starting point. We always begin in the middle somewhere, in a context that has yet to be fully elaborated. The work of elaboration is itself the answer. (See also Interpretation.)