So that perhaps, how clear an idea soever we think we have of the extension of body, which is nothing but the cohesion of solid parts, he that shall well consider it in his mind, may have reason to conclude, That it is as easy for him to have a clear idea how the soul thinks as how body is extended.

For, since body is no further, nor otherwise, extended, than by the union and cohesion of its solid parts, we shall very ill comprehend the extension of body, without understanding wherein consists the union and cohesion of its parts; which seems to me as incomprehensible as the manner of thinking, and how it is performed.
We can as little understand how the parts cohere in extension as how our spirits perceive or move.
25.

I allow it is usual for most people to wonder how any one should find a difficulty in what they think they every day observe.

Do we not see (will they be ready to say) the parts of bodies stick firmly together?
Is there anything more common?
And what doubt can there be made of it?
And the like, I say, concerning thinking and voluntary motion.

Do we not every moment experiment it in ourselves, and therefore can it be doubted?
The matter of fact is clear, I confess; but when we would a little nearer look into it, both in the one and the other; and can as little understand how the parts of body cohere, as how we ourselves perceive or move.