The order of ideas and things is the same
Formal Statement
The order and connection of ideas is the same as the order and connection of things. A mental event and its corresponding bodily event are not two causally linked happenings but one and the same event described under two attributes — Thought and Extension.
In Plain Language
Think of a single coin with two faces. Your thought and the brain-state that "is" that thought are not cause and effect — they are the same reality read in two languages. Whatever pattern you find in the physical world has an exact mirror in the world of ideas, because both are expressions of one substance.
Why This Follows
This is our starting point. Spinoza proved in Part I that there is only one substance expressed through infinite attributes. Part II, Proposition 7 draws the direct consequence: since Thought and Extension are attributes of the same substance, their internal orderings must be identical.
Mind and body are parallel expressions of one reality, never interacting as separate domains.
If mind and body are truly the same event under two descriptions, why do we so persistently experience them as separate?