Conceptual Metaphors frequently have an abstract Source Domain. That's especially true when the Target Domain is also abstract.
Consider the common Love is a Journey. That's Metaphor System with a generic source domain. It allows quite a variety of individual metaphorical statements, like:
* I see no path forward. [Traveling on foot...] * Our relationship is foundering [... on a ship] * We're spinning our wheels. [... or a car]
These are possible because the source domain is abstract. You can choose any vehicle to illustrate something about love.
The Source Domain is especially likely to be abstract if the Target Domain. Contrast Love Is a Journey to Paintbrush As Pump.
Lakoff and Johnson, p. 45: "Once again, there is no single consistent image that the Journey metaphors all fit. What makes them *coherent* is that they are all Journey metaphors, though they specify different means of travel."
See also: Monad As Metaphor