Hence, although we seem to ourselves to know the external world as it is, our knowledge can never extend beyond the impressions made upon us.

Thus, we are absolutely shut up to _appearances_, and can know nothing about the _reality_ to which they must be referred.
Touching this matter Herbert Spencer writes[1] as follows: "When we are taught that a piece of matter, regarded by us as existing externally, cannot be really known, but that we can know only certain impressions produced on us, we are yet, by the relativity of thought, compelled to think of these in relation to a cause--the notion of a real existence which generated these impressions becomes nascent.

If it be proved that every notion of a real existence which we can frame is inconsistent with itself,--that matter, however conceived by us, cannot be matter as it actually is,--our conception, though transfigured, is not destroyed: there remains the sense of reality, dissociated as far as possible from those special forms under which it was before represented in thought." This means, in plain language, that we must regard everything we know and can know as appearance and must refer it to an unknown reality.
Sometimes Mr.Spencer calls this reality the Unknowable, sometimes he calls it the Absolute, and sometimes he allows it to pass by a variety of other names, such as Power, Cause, etc.

He wishes us to think of it as "lying behind appearances" or as "underlying appearances." Probably it has already been remarked that this Unknowable has brought us around again to that amusing "telephone exchange" discussed in the third chapter.

But if the reader feels within himself the least weakness for the Unknowable, I beg him to consider carefully, before he pins his faith to it, the following:-- (1) If we do perceive external bodies, our own bodies and others, then it is conceivable that we may have evidence from observation to the effect that other bodies affecting our bodies may give rise to sensations.