This conception of causal nature is not to be confused with the distinct conception of one part of nature as being the cause of another part.

For example, the burning of the fire and the passage of heat from it through intervening space is the cause of the body, its nerves and its brain, functioning in certain ways.

But this is not an action of nature on the mind.

It is an interaction within nature.

The causation involved in this interaction is causation in a different sense from the influence of this system of bodily interactions within nature on the alien mind which thereupon perceives redness and warmth.
The bifurcation theory is an attempt to exhibit natural science as an investigation of the cause of the fact of knowledge.