P12: The images of things are more easily joined to images related to things we understand clearly and distinctly than to other images.

Dem.: Things we understand clearly and distinctly are either common properties of things or deduced from them (see the Def. of reason in IIP40 S2), and consequently (by P11) are aroused in us more often. And so it can more easily happen that we consider other things together with them rather than with {things we do not understand clearly and distinctly}. Hence (by IIP18), {images of things} are more easily joined with {things we understand clearly and distinctly} than with others, q.e.d.