Middle Way Philosophy did not come out of nowhere, even if that’s how it might look when it is presented as simply as possible. Nevertheless, crediting all the sources that have contributed to it is a complex process, only done thoroughly in Robert M Ellis’s first book (his Ph.D. thesis), A Theory of Moral Objectivity. That thesis contains a wide survey of Western thinkers, and tries to separate out in each what is helpful and unhelpful to the Middle Way. Most people, though, will not want to go into it in this much detail.
The following resource, The Sources of Middle Way Philosophy (click ‘+’ on bar that comes up at bottom to enlarge diagram), is an attempt to represent some of the main inspirations for Middle Way Philosophy clearly in diagrammatic form. As you will see, there are some roots in Buddhism, and particularly the thought of Sangharakshita. Popperian thought and the philosophy of science is also crucial to the conception of objectivity. George Lakoff’s embodied mind thesis is important. There are also elements of some mainstream Western philosophers such as Aristotle, Hume and Kant, but like all philosophers these are used very selectively, not wholesale. The psychoanalytic tradition, particularly Jung, is also crucial in providing the integration model.
There is no one thinker who completely encapsulates the Middle Way, so any Middle Way Philosophy is bound to be a magpie’s nest of influences collected from different sources. Nevertheless, it is also very far from being a set of random claims appearing out of a vacuum. Like any kind of thought, it has a genealogy.