304) As a result of this...

However, this necessary assent indicates only the existence of an external reality of the world in general. But the objective reality of every sense perception is not known in a necessary manner. Therefore, we need evidence to prove the objectivity of every specific sense perception. This evidence is the principle and laws of causality.

The occurrence [in the senses] of the form of a specific thing under specific circumstances and conditions reveals, in accordance with this principle, the existence of an external cause of that thing. Were it not for this principle, sense perception or the presence of a thing in the senses could not reveal the existence of that thing in another sphere.

Because of this, in a specific case of illness, a human being may perceive certain things, or imagine that he sees them, without discovering an objective reality of those things. This is because the principle of causality does not prove the existence of that reality, as long as it is possible to explain a sense perception by the specific case of the illness.

Rather, it proves the objective reality of sense perception if there is no explanation of it in light of the principle of causality except by an objective reality that produces the sense perception. From this one can draw the following three propositions. First, 'Sense perception by itself does not disclose the existence of an objective reality, since it is conception, and it is not the task of conception (regardless of its kind) to give a true disclosure'.

Second, 'Knowledge of the existence of a reality of the world in general is a necessary and primary judgement that does not require evidence; that is, it does not require prior knowledge'- this is the point separating idealism from realism. (p. 305) Third, 'Knowledge of an objective reality of this or that sense perception is acquired only in light of the principle of causality'.