The countercultures of the 1960s-80s took attitudes to boundaries as their central themes. The monist counterculture—the 1960s youth movement—wanted to eliminate all boundaries and level all distinctions; the dualist counterculture, or religious right, wanted to make them absolute.
Monism and dualism are opposites. But because each is obviously wrong, each turns into the other when cornered. A devious trick!
Monism is the stance that fixates sameness and connections, and denies differences and boundaries. Dualism is just the other way around: it denies sameness and connections, and fixates differences and boundaries.
Both these confused stances sometimes show themselves to be obviously wrong. The complete stance of participation recognizes that samenesses and differences, boundaries and connections, are all real, but also always somewhat nebulous: ambiguous and fluid. This is obviously accurate, but usually less convenient. Monism and dualism are simpler, and deliver particular emotional payoffs—some of the time.