Furthermore there is nothing else in our knowledge analogous to these causes which influence the mind to perception.

Accordingly, beyond the rashly presumed fact that they occupy time, there is really no ground by which we can determine any point of their character.

They must remain for ever unknown.
Now I assume as an axiom that science is not a fairy tale.

It is not engaged in decking out unknowable entities with arbitrary and fantastic properties.

What then is it that science is doing, granting that it is effecting something of importance?
My answer is that it is determining the character of things known, namely the character of apparent nature.
But we may drop the term 'apparent'; for there is but one nature, namely the nature which is before us in perceptual knowledge.