Similarity (psychology)
One important tool used by psychologists to look into similarity is computer modeling.  Scientists will, for example, ask people to rate a number of objects as more or less similar to each other.  Then they will write a computer program that tries to simulate the same answers that the humans gave.  For instance, a person might say that "cat" and "dog" are similar words, so a good computer model should be able to take the word "dog" and respond with the word "cat" in a list of similar words.  Computer models do this by applying some set of rules to a large collection of real world text (called a corpus).