The concept of "like is known by like" was a major philosophical doctrine at least as early as the 5th century B.C.E. Pythagoras taught that the extent or depth of our knowledge of the divine depends on us being like the divine, or assimilating to the divine. The idea was that to the degree that we have knowledge of the divine, we must have changed our own character from human to divine.

The concept of "like is known by like" was a major philosophical doctrine at least as early as the 5th century B.C.E. Pythagoras taught that the extent or depth of our knowledge of the divine depends on us being like the divine, or assimilating to the divine. The idea was that to the degree that we have knowledge of the divine, we must have changed our own character from human to divine.

In general, it's meaning is simple and intuitive - the more similar a person is to oneself, the easier it is to get to know or understand that person.A second but related meaning of the phrase is knowing through analogy. This usage goes back at least to the Presocratics, especially Empedocles.