For a common man, the independent existence of the world outside the mind, and thus the existence of the glowing sun, hard ground, cold water and the like, is beyond the slightest doubt; he holds this as the unshakable truth. It is enough, however, only to think for a while, to see that there is something even more certain, namely, that there is my own consciousness, because if it were not so, I would know nothing about the material world. Enough is to pronounce this sentence to be immediately convinced that my own consciousness can be the only appropriate starting point for any philosophical speculation. It is amazing that so many centuries had passed before people managed to understand it. Only Descartes with his famous cogito, ergo sum considered the consciousness as the starting point.
Consider the first manifestation of our consciousness. The first of its content can be, of course, nothing but a feeling of a different kind: the feeling of light, sound, pain, pleasure, etc. These feelings come to consciousness, go out of it and change without our complicity. They are the only content of our mind; the content that presents itself to us not as something created by the consciousness, but as something imposed.
The consciousness then assumes that there is an external object, the presence of which is necessary to induce feelings. But this assumption of the existence of external objects, which apparently takes place without any consideration and thought, is actually a logical conclusion, without which we would never have come to know the outside world.
It is obvious therefore, based on the above analysis, that the material world can not be regarded as a collection of some real beings, independent of our mind, which could exist even when the mind did not think or imagine anything.
Adolf Eugen Fick – Die Welt als Vorstellung. Akademischer Vortrag