Modern secular thought has its own dualism: It treats only the physical world as knowable and testable, while locking everything else - mind, spirit, morality, meaning - into the realm of private, subjective feelings. The so-called fact/value split.
Male and female represent the two sides of the great radical dualism. But in fact they are perpetually passing into one another. Fluid hardens to solid, solid rushes to fluid. There is no wholly masculine man, no purely feminine woman.
In the first place, Descartes stands for the most explicit and uncompromising dualism between mind and matter.
I believe in general in a dualism between facts and the ideas of those facts in human heads.
The dualism itself becomes a sort of presupposition or datum; its terms condition the further problem.