The Middle Way is a concept which is most clearly expressed in the life and teachings of the Buddha, the founder of Buddhism who lived about 2500 years ago in northern India. However, I want to argue that it is a universal concept that has been recognised and practised to varying degrees in different places.
In the Buddha’s account, the Middle Way involves the avoidance of two extremes, eternalism and nihilism (or annihilationism). This is first shown in the Buddha’s life history, in which he is depicted as moving towards enlightenment by avoiding, first nihilism, then eternalism: see the Buddhist origins of the Middle Way for details. Both these extremes involve metaphysical beliefs (or their denial) that the Buddha avoided speculation about in his ‘silence’ about four unanswerable questions (the eternality and infinity of the universe, the existence of the self after death, and the existence of the Buddha after enlightenment). See this page for a list of key references for the Middle Way in Buddhist scriptures.
In the traditional Buddhist account, however, eternalism is primarily interpreted as belief in an eternal self, and nihilism a denial in the eternal self (the belief that one is annihilated at death). These views are, however, also linked to moral attitudes of absolutism v relativism, and later by Nagarjuna to beliefs about the ultimate existence of the world or its non-existence. I think the traditional Buddhist account of the Middle Way is too narrow, because it takes the metaphysical beliefs that were most important in the Buddha’s time (especially ones about reincarnation of the self) as the only ones that need to be avoided, rather than interpreting the Buddha’s ideas as showing the general need to avoid all metaphysical beliefs or their denial (see this page for details). The Buddha’s insights are only useful today if they deal with today’s common beliefs.
In my earlier work, such as A Theory of Moral Objectivity, I tried to update eternalism and nihilism and apply them to Western philosophies. However, I now think that the energy put into defending specific views of eternalism and nihilism was not well spent, because the distinction between them is not important. The underlying, important, and universal point offered by the Middle Way is that metaphysical beliefs and their denial need to be avoided in order to make progress in addressing conditions. It is not just that metaphysical beliefs are a bit of a distraction: they are actually damaging because they rigidify our positions and stop us responding to new information in experience.
The definition of a metaphysical belief, however, is a matter of controversy. I accept that in practice, the borderline of when someone holds a metaphysical belief is vague, but in the abstract, metaphysics defines itself quite precisely. I think the best indicator is that a metaphysical belief cannot be incrementalised (made a matter of degree) – it is an all-or-nothing belief, otherwise known as a dualism. See this page for more details on defining metaphysics. Some classic examples of metaphysical beliefs are: that God exists (or does not); that there is absolute value of any kind (or there is not); that we have a fixed self (or not); that there is freewill or determinism; that the universe is real or only in our minds. Some of these are assumptions widely used in everyday judgement: for example, we can try to avoid responsibility by assuming determinism.
So, the prime meaning of the Middle Way as understood in Middle Way Philosophy is the avoidance of both poles of metaphysical belief or its denial, whatever the nature of the metaphysical belief. To avoid either believing in or denying these extremes, we need to adopt a position of metaphysical agnosticism. This means a clear and deliberate refusal to adopt (or even to slip into) either position, which is often quite difficult if people from different groups are pressurising you to show solidarity with the beliefs of their group. Maintaining this agnosticism, though, we can avoid some of the grosser kinds of attachment that stop us from integrating and becoming more objective. Of course, we still then have to deal with more subtle kinds of metaphysics. If you probe a rigid position, there is usually a metaphysical belief there somewhere, and a bit of critical thinking (as well as, perhaps, meditation and other practices) can help dislodge it. The most we can probably hope for is to loosen the grip of metaphysics in our lives rather than removing it altogether.