in terms of their validity, and that there is no place left for the application of the principle of economy (apart from certain very special cases in still unfinished areas of science). We can see also that the 
establishment of this order points the way to the basis of which we are in search. 

v 

Here of course the greatest care is necessary. For we are treading on the path which has been followed from ancient times by all those who have ever embarked upon the journey towards the ultimate grounds of truth. And always they have failed to reach the goal. In the ordering of statements according to their origin which I undertake for the purpose of judging their certainty, I start by assigning a special place to those that I make myself. And here a secondary position is occupied by those that lie in the past, for we believe that their certainty can be impaired by "errors of memory" and indeed in general the more so the farther back in time they lie. On the other hand, the statements which stand at the top, free from all doubt, are those that express facts of one's own "perception," or whatever you like to call it. But in spite of the fact that statements of this sort seem so simple and clear, philosophers have found themselves in a hopeless labyrinth the moment they actually attempted to use them as the foundation of all knowledge. Some puzzling sections of this labyrinth are for example those formulations and deductions that have occupied the center of so many philosophical disputes under the heading "evidence of inner perception," "solipsism," "solipsism of the present moment," "self-conscious certainty," etc. The Cartesian cogito ergo sum is the best-known of the destinations to which this path has led -- a terminating point to which indeed Augustine had already pushed through. And concerning cogito ergo sum our eyes have today been sufficiently opened: we know that it is a mere pseudo-statement, which does not become genuine by being expressed in the form "cogitatio est" -- "the contents of consciousness exist."" Such a statement, which does not express anything itself, cannot in any sense serve as the basis of anything. 

It is not itself a cognition, and none rests upon it. It cannot lend certainty to any cognition. 

There exists therefore the greatest danger that in following the path recommended one will arrive at empty verbiage instead of the basis one seeks. The critical theory of protocol statements originated indeed in the wish to avoid this danger. But the way out proposed by it is unsatisfactory.