Augustine connects this with the problem of certainty. He says that we have immediate evidence of two things, namely, the logical form – because even the question of evidence presupposes the logical form – and secondly, the immediate sense experience, which should really be called sense impression because” experience'” is too ambiguous. What he means is this; I now say that I see blue. The piece of color may objectively be not blue but green – I sometimes confuse these two, especially in female dresses, (the horror of Mrs. Tillich!) – in any case, I now have blue, as sense impression. This is absolutely certain, even if the dress is not blue. Now this is what he means with immediacy. I see a man, but I come nearer and it is a tree, in reality; this often happens when you walk through a fog and cannot distinguish a man from a tree, if they are a little bit away from us. This means there is no certainty about the objective element in it, but there is absolute certainty about the impression I have as such. This means there is skepticism about everything real. Logical forms are not real; they are structures which make questions possible; therefore they are immediate and necessary.
Secondly, sense experiences are not real. They are real only insofar as I have them.
But whether they are more than this, I don’t know. Therefore these two evidences – of the logic and of the perception – do not overcome skepticism.
Now how can doubt about reality be overcome? You must start with the general doubt. You must doubt about everything. It was not Descartes who said this first. It was not even Augustine, but Augustine also said it. Therefore, is there a point of certainty, somewhere? He says: “You know that you are thinking.” “I know.” “Do not go outside; go into thyself” – namely where you are thinking – “The truth dwells in the interior of man, for a mind knows nothing except what is present to the mind. But nothing is more present to the mind than the mind itself.” i. e., the immediate self-consciousness of the asking skeptic is the fixed point.. The truth which was lost in the exterior world, where everything fell under doubt, is found again in the interior world. The soul is the inner realm, in contrast to Greek philosophy, in which it is the power of life. The discovery of soul, in this sense, is one of the most important consequences of Christianity. It includes the world as the sum of all appearances. In contrast to the Greeks, where the soul is a part of all things, the world is an object. Now the world is an appearance for the soul, which is the only real thing.
Now these ideas – Go into thy inner reality and there you will find truth – sound very much like Descartes’ cogito ergo sum (I think, therefore I am). But the difference is that in Deseartes, the self-certainty of the ego is the principle of mathematical evidence – he derives from this his rational system of nature – while for Augustine the inner evidence is the immediacy of having God. So he says, after saying “go into thyself,” “And after you have your soul immutable, transcend yourselves i. e., in your soul is something which transcends your soul, something immutable, namely, the Divine Ground. It is the immediate awareness of that which is unconditional, to which he refers here. This is certainly not an argument for the existence of God, but it is a way of showing that God is presupposed in the situation of doubt about Him. “While not seeing what we believe, we see the belief in ourselves.” i. e. , we see the situation of being grasped by something unconditional.