A person should speak only of what he knows and  not make statements about something he does not know

The opinion that there are limits to human cognition that cannot be overstepped, compelling man to stop short before an invisible world, must be replied to by saying that there can be no doubt about the impossibility of finding access to the invisible world with the kind of cognition referred to here. Whoever considers that form of cognition to be the only possible one cannot come to any other opinion than that the human being is denied access to a possibly existent higher world. 

Yet the following may also be stated. If it is possible to develop another kind of cognition, this then may well lead into the supersensible world. If this kind of cognition is considered to be impossible, then we reach a point of view from which all talk about a supersensible world appears as pure nonsense. From an impartial viewpoint, however, the only reason for such an opinion can be the fact that the person holding it has no knowledge of this other kind of cognition. Yet how can a person pass judgment upon something about which he himself admits his ignorance? 

Unprejudiced thinking must hold to the premise that a person should speak only of what he knows and  not make statements about something he does not know. Such thinking can only speak of the right that a person has to communicate what he himself has experienced, but it cannot speak of the right that somebody declare impossible what he does not know or does not wish to know. We cannot deny anyone the right to ignore the supersensible, but there can never be any good reason for him to declare himself an authority, not only on what he himself can know, but also on all that a man can not know.

Source: Rudolf Steiner – GA 13 – An Outline of Occult Science: I: THE CHARACTER OF OCCULT SCIENCE

Translated by Maud and Henry B. Monges and revised for this edition by Lisa D. Monges