A major problem faced by dualists is the inability to resolve the rift created between the two opposing elements. Typically the motivation for resolving conflicts between these two realms is to make the world more understandable. Descartes, for example, claimed that the pineal gland is the point of contact between the bodily and spiritual realm. The inability to rectify these two realms has inclined some to adopt monism. Mental events are reduced to brain states, thus leaving only the bodily realm, thus monism.
Parallelism contends that the mind and body interact independently of one another.
Thus having separate existences, they have no causal connection and have no interaction. Epiphenomenalism contends there is only a one-way causal connection from the body to the mind, but none from the mind to the body. Consciousness is just a byproduct of the body, much like smoke from a steam engine train. Others attempt to salvage dualist theories by switching elements within the mind or body. The only gap we have from this distinction is epistemological rather than ontological.