From the possible combinations of these five fundamental forms of error, three more complex ones are derived: dualism, when two contradictory methods, one logically legitimate and the other illegitimate, or both equally false, are brought together, and considered to be both philosophically valid; scepticism, when the mind, in the presence of confusion and error, asserts the mystery of reality, which is the problem itself, but denies its own power to deal with it; and finally, mysticism, when even that last semblance of thought, by which the sceptic affirms that there is a mystery, is abandoned, and the immediate actuality of life is regarded to be the only truth. Dualism leads inevitably to the conception of a double reality, and we have already seen how the whole of Croce's speculation continually tends towards the logical unification of dualities, as with spirit and nature, value and fact. Every philosophical problem seems to present itself to his mind as involved in a dualistic difficulty; every solution becomes satisfactory to him only when the last shreds of dualism are eliminated from it. While scepticism is a logical error (the affirmation of a purely negative position), it contains within itself one of the essential moments of every progress in thought, the scepsis, or philosophical doubt, which is the negation of an error, and therefore the germ of every true affirmation. As for mysticism, we have dealt with it elsewhere as being one of the untenable aspects of logical scepticism; we may add that, if it ever obeyed the laws of internal coherence, we should not even be able to discuss it, since its only conceivable expression would be an ecstatic silence. The same character of necessity that invests these forms of the logical error is present also in the false solutions of other philosophical problems, and we need only refer the reader to our discussion of æsthetic theories. In both cases, not only the number, but also the logical succession, of the necessary forms of error, depends on the number of possible arbitrary combinations of the spiritual forms, or concepts of reality. But infinite, on the other hand, are the individual forms of error, as infinite are the individual forms of truth: the problems are always historically conditioned and variable, and so are also the solutions and the false solutions, determined by feelings, passions, and interests.