Each attribute is conceived through itself
Formal Statement
Each particular attribute of the one substance must be conceived through itself. An attribute is what the intellect perceives as constituting the essence of substance (Def.4), and substance is conceived through itself (Def.3). Therefore each attribute, as an expression of substance's essence, must likewise be conceived through itself.
In Plain Language
This proposition does critical work: it guarantees that attributes are conceptually independent of one another, even though they all belong to one and the same substance. Thought does not explain Extension, and Extension does not explain Thought — each is intelligible on its own terms. This is what later allows Spinoza to reject mind-body interaction without falling into dualism. The attributes are distinct ways of understanding one reality, not separate realities.
Why This Follows
By Def.4 (gs-02), an attribute is what the intellect perceives as constituting substance's essence. By Def.3 (gs-01), substance is conceived through itself. So the attribute, which just is substance's essence as the intellect grasps it, must also be conceived through itself — there is nothing external to mediate the conception.
Attributes are conceptually independent of one another, even though they all belong to one substance.
If each attribute is conceived through itself, how can they all belong to the same substance without that substance being fragmented?