A rough translation of MOTOORI Norinaga's "comments about the principle of yin and yang made while answering a question from MINAMIKAWA "Bunboku" Kinkei (南川金渓) about Shintō", from the Suzuya tōmon roku:

First, there are many things in our world that are paired, such as heaven and earth, sun and moon, man and woman, day and night, and water and fire. The existence of so many such pairs is often attributed to the principle of yin and yang, but this is most certainly not the case. It is a natural phenomenon. Consider: If you have one thing, and add another thing, you have two things. Or if you have one thing and divide it once, it becomes two things. Because this is the case, we would expect many things to occur in twos. In fact, things that occur singly are even more common than pairs; people simply do not notice them.

To use the human body as an example: Ears, eyes, hands, feet, and the like are paired, but heads and noses and mouths and navels are not. If the yin/yang principle were true, we would expect everything to occur in twos. Since this is not the case, we can conclude that there is no particular reason that some things occurring singly, others in twos, and some rare things even in threes: they just do. [...]

It is true that things that occur in twos often seem to be in opposition, but this has nothing to do with yin and yang either. Consider: A pair is only a pair if it includes a "this" and a separate "that". [...] But when such an arrangement is considered in isolation, with no context, the difference between "this" and "that" cannot but be an opposition. It is also commonplace for a thing that does not necessarily appear in pairs to be associated with another thing in order to form a pattern of opposition. The idea of yin and yang is in every respect an empty one, a term invented simply to be applied to things occuring in twos.

Of course this hard-headed critical thinking would be a lot more impressive if it weren't floating in a sea of Kojiki-based explanations of which kinds of kami can die and which kind can't, why it makes no sense for humans to go anywhere but Yomi when they die, and so on.

Update! More Motoori (on an unrelated topic) at BMSF.