But from a clear insight into the nature of these two questions, there appears to me to follow a second most impor tant result: namely, what is the nature of the evidence, by which alone any ethical proposition can be proved or disproved, con firmed or rendered doubtful. Once we recognise the exact meaning of the two questions, I think it also becomes plain exactly what kind of reasons are relevant as arguments for or against any particular answer to them. It becomes plain that, for answers to the first question, no relevant evidence whatever can be adduced: from no other truth, except themselves alone, can it be inferred that they are either true or false. We can guard against error only by taking care, that, when we try to answer a question of this kind, we have before our minds that question only, and not some other or others; but that there is great danger of such errors of confusion I have tried to shew, and also what are the chief precautions by the use of which we may guard against them. As for the second question, it becomes equally plain, that any answer to it is capable of proof or dis proof — that, indeed, so many different considerations are relevant to its truth or falsehood, as to make the attainment of proba bility very difficult, and the attainment of certainty impossible. Nevertheless the kind of evidence, which is both necessary and alone relevant to such proof and disproof, is capable of exactdefinition. Such evidence must contain propositions of two kinds and of two kinds only: it must consist, in the first place.