Similarity and contiguity are the two most elementary relationships that the mind recognizes between two real or mental objects. Given two objects O1 and O2, then
- O1 and O2 bear a relationship of similarity to each other if they have a set of properties in common by which they can be subsumed under the same category. They will differ by other properties; so the relationship of similarity includes contrast. This is a virtual relation constructed by comparing O1 and O2. It does not matter whether O1 and O2 bear a physical relation – i.e., one of contiguity – to each other (more probably not).
- O1 and O2 bear a relationship of contiguity to each other if O1 is physically connected with O2. This includes spatial contiguity, temporal simultaneity or immediate succession and a causal relation. This is an actual relation that the mind recognizes in copresent O1 and O2. It does not matter whether O1 and O2 bear a relationship of similarity to each other (more probably not).