We may mingle, and unite, and separate, and confound, and vary our ideas in a hundred different ways; but until there appears some principle, which fixes one of these different situations, we have in reality no opinion: And this principle, as it plainly makes no addition to our precedent ideas, can only change the manner of our conceiving them.
All the perceptions of the mind are of two kinds, viz.

impressions and ideas, which differ from each other only in their different degrees of force and vivacity.

Our ideas are copyed from our impressions, and represent them in all their parts.

When you would any way vary the idea of a particular object, you can only encrease or diminish its force and vivacity.

If you make any other change on it, it represents a different object or impression.