Synthesis is the intercourse of knowledge. The real aspersion in the Garden of Eden was this gift to collect and couple of bits of knowledge to sire new ideas. This craft is (perhaps) uniquely human. It is why the storing and sharing of knowledge consumes much of our human endeavor. Without knowledge there is no synthesis.
How our brains perform synthesis is undiscovered. Ask someone how it is that they came up with a brilliant idea, and they shrug. It just happens. And like most human abilities, some humans are more gifted at it than others.