Of those three forms of logical scepticism, æstheticism, mysticism, and empiricism, the third one leads us to the distinction between the logical concept and the scientific concepts, or fictions. The logical or pure concept is beyond all individual representations, and must therefore not contain any particular representative element; but, on the other hand, being the universal as opposed to the individuality of representations, it must refer to all and each of them. If we think, for instance, of the concepts of beauty, truth, quality, development, and such like, it will be impossible for us to represent or imagine a sufficiently large fragment of reality that will exhaust them, or such an infinitesimal one as will not admit them. This is what is meant by saying that the concept is at the same time universal and concrete, or, in other words, that it is transcendent in respect to every single representation, and yet immanent in all of them. A third characteristic of the pure concept, besides those of universality and concreteness, is that of expressivity: being a product of knowledge, it must be expressed and spoken, and cannot be a dumb act of the mind, such as practical acts are. The conceptual fictions, or, as Croce called them on account of their non-theoretical character, the pseudo-concepts differ from the pure concept in being either concrete and representative but not universal, or universal without any possible reference to individual representations, that is, without concreteness. The first class is that of empirical concepts, which contain some objects or fragments of reality, but not the whole of reality: such as the concepts of house, cat, rose. The second is that of abstract concepts, which contain no object or fragment of reality: such as those of triangle in geometry or of free movement in physics. The first are real, but not rigorous, the second rigorous, but unreal. Neither the ones nor the others can be considered as mistaken concepts or errors, since after having criticised them from a logical point of view, we still continue to use them for what they are; nor as imperfect concepts, and preparatory to the perfect ones, since their formation presupposes the existence of the perfect and rigorous ones: it would be impossible to conceive the house, the rose, the triangle, before conceiving quantity, quality, existence, and other pure concepts. It is true that in the actual development of thought, conceptual fictions have again and again given birth to true concepts; but in that case they have lost their intrinsic nature, and have assumed the characters of the genuine logical activity. In order to understand the proper function or nature of the conceptual fictions, it is necessary to fix our attention on the moment of their formation, which is practical and not logical. Their justification lies in their practical end and in their usefulness: they are instruments by the help of which we can recall with a single word