Attribute
Formal Statement
By attribute, I mean that which the intellect perceives as constituting the essence of substance. An attribute is not something added to substance from outside; it is how substance's essence shows up for an intellect.
In Plain Language
Think of an attribute as a fundamental lens through which the nature of substance becomes intelligible. Substance does not hide behind its attributes — each attribute fully expresses what substance is, just in a different fundamental "register." Spinoza will later argue that there are infinitely many such registers, though we humans know only two: Thought and Extension.
Why This Follows
Having defined substance as self-conceived (gs-01), we need a term for the essential character that the intellect grasps when it conceives substance. The attribute is that character.
An attribute is what the intellect perceives as constituting the essence of substance.
Connected Concepts
Is an attribute something substance "has," or is it something substance "is"? What difference would it make?