The only doctrine capable of truly reconciling the spirit-matter duality is also the doctrine which reconciles all particular dualities, even the more universal, such as the one made up of ‘essence’ and ‘substance’. This doctrine does not have a name in Western philosophy, which is to be expected since it is of the metaphysical order, as it must be in order to achieve this reconciliation. In Sanskrit the doctrine is called advaita-vada. In English, the most accurate translation is ‘non-dualism’ or ‘the doctrine of non-duality’. The only misfortune here is that the term, in the first form given, ends in ‘-ism’ which gives the impression that it is but another system offered along with all the others, which is certainly not the case, being but the name of a metaphysical principles of the highest order. The name itself warrants comment.
As principles become more universal, it becomes more difficult to speak of them affirmatively. For example, the term Infinite is, technically speaking, a negation meaning ‘without limits.’ But in another sense, this way of speaking amounts to a ‘negation of negation,’ since although it is stated in the negative, it amounts to a universal affirmation. The same can be said for ‘non-duality’, which is the negation of duality, and which, being a ‘negation of negation’, amounts to a more universal and open-ended affirmation than the one made by monism, which simply states that all is one. As regards the spirit-matter duality in particular, the difference between non-dualism and monism is that the former does not claim that one of the two terms is reducible to the other, while still denying that their opposition is irreconcilable. It reconciles them, but it does so by acknowledging an order that is beyond them, and there it locates their common principle. By adopting this point of view, non-dualism acknowledges the distinctness of the terms, and even their opposition, within a certain order, while also allowing for their unity through the principle.
To summarize and clarify using the human body as an illustration, we might say that on the level of philosophy, the body has a right hand and a left hand, and that philosophy either stops there, treating them as separate, irreconcilable beings, or else it adopts the ‘monist’ view and treats them both as left arms or both as right. Non-dualism alone is able to step back far enough to see that they actually belong to one body, and are, even though distinct and opposed at a certain level, still reconciled into a single unity. It is precisely this unity that is beyond the ken of rationalist philosophy.