One of the most obviously incoherent aspects of modern thought is the presence, at the same time, of both voluntarism and materialism/naturalism/scientism. These things would not seem to go together well at all. The voluntarism suggests that it is our own wills which define reality. If I say I am a woman, even if I am a man, then that is what I am and I should be treated as such by society. This conflicts with the materialism/naturalism/scientism which sees reality in terms of material processes. According to this outlook it would be genetics, chromosomes and hormones and such like that would determine my sex.
Many moderns hold to both voluntarism and scientism with equal force, despite the apparent incompatibility. How can we explain this? I don't personally have a modern type mind, so cannot answer with confidence, but I can suggest three possible explanations.
It can be the case that certain philosophies influence a culture over the course of that culture's history. Instead of these philosophies being harmonised, they simply "enter the mix". If this is the explanation, then the voluntarism might come from a variety of sources, e.g. from the theological voluntarism of the Middle Ages, or from German idealist philosophy of the nineteenth century, or more generally from the emphasis on autonomy as the goal of a liberal politics. The scientism/materialism/naturalism is derived from the rejection of scholastic philosophy in the Early Modern period and perhaps from empiricist schools of philosophy.
b) Science as a servant of human desires
My understanding is that modern science was launched, in part, with the idea that by understanding natural processes, humans could obtain the resources to satisfy unlimited wants. In other words, if the larger aim is not to live within the natural order, but to pursue our individual wants and desires to the furthest extent possible, then science could be employed to create the conditions in which those wants and desires could be fulfilled.
If this is so, then you can understand why moderns cleave to both scientism and voluntarism. The voluntarism represents the unfettered pursuit of whatever we will for ourselves. The scientism the means by which to obtain these wants and desires.
c) The loss of value in nature
If nature is seen only from a scientistic/naturalistic viewpoint, then it will seem merely mechanical. It will no longer be a bearer of value in the way it once was when it was appealed to morally (i.e. when saying "it is natural/unnatural to do x, y or z" as a way of endorsing or condemning certain acts).
I think it can be difficult for those raised within a Christian tradition to understand this. Christians are used to the idea of a purposeful act of creation, so that our relationship to the natural world is invested with meaning (even when we apprehend a certain mystery in the created world). But there are moderns for whom nature is just a mechanical process coldly indifferent to human life. There is nothing for them to relate to in the natural world.
So values, for such moderns, must then come from ourselves: they must come from our own subjective wills. We do not discover objective values inhering in the created world; instead, we assert the power to create values through an act of will (which perhaps represents a deification of ourselves in the image of a voluntarist concept of God).
You can see, then, why the scientism/naturalism/materialism goes together with a voluntarism. The scientism disenchants and de-values; the voluntarism is then necessary to reassert value. You get both, despite an apparent incompatibility between the two.
I'm not sure which of the three explanations is the more likely reason for the coexistence of both voluntarism and scientism. Perhaps all have had an influence, or there might be some other reason I have not considered.