The distinction between appearance and reality is drawn here just as it is drawn in the world of our common everyday experiences.
The great majority of the touch things about us we are not actually touching at any given moment.

We only _see_ the things, _i.e._ we have certain _signs_ of their presence.

None the less we believe that the things exist all the time.

And in the same way the man of science does not doubt the existence of the real things of which he speaks; he perceives their _signs_.

That certain experiences are to be taken as signs of such realities he has established by innumerable observations and careful deductions from those observations.