What is the difference between substance and object?

Answer 1

This depends on what you mean by substance and object. In today's colloquial speech, substance usually means chemical substance. "Object" is merely a synonym for "thing". Understood this way, the relationship between object and substance then is that objects are made of one or more substances.

However, there are a few meanings of substance. The term first appears on the scientific landscape in philosophy and the philosophical definition is the canonical intellectual meaning. Aristotle, for instance, calls substances any unity of form and matter, which is called hylomorphism (for him, only living things were true substances, as opposed to technological things). You, for example, are a substance. Substances are tricky because on the one hand, they are those things which persist through change (you can grow and change, but you remain the same person), but on the other hand, properties that can change without destroying a thing change because the substance changes. So in the first case, substance is a bit like an unchangeable core of what a thing is, on the other hand it's the entire thing. I don't want to get too deep into the metaphysics, but the important thing to remember is that each living thing for Aristotle is a substance, a single thing and not just a heap of other stuff. (For some atomists, atoms are the only true substances, everything else being something like a pile of atoms.)

As I said, form and matter are what compose substance. Form is that which makes a thing what it is. Matter, however, here doesn't mean what we mean today (today, matter, oddly enough, is what chemical substance means today, or what secondary matter means for Aristotle). It means the principle by which form is manifested materially. Forms cannot exist by themselves; you can only have things which are enformed. Form needs matter and this unity is what makes things exist as objects. Thus, objects are things and substances are a subclass of things.

Answer 2

Substance refers to the underlying nature or essence of something, while object is a specific thing that can be perceived by the senses. In philosophical terms, substance is often considered to be the fundamental reality that underlies all objects.