misunderstood Alice's statement that she "saw nobody on the road" he was expressing the curious view that the person Nobody is the person who is there when no visible or tangible person is there. This raises a laugh; but its "absurdity" is of the same kind as that perpetrated by primitive peoples and theologians who say that your immortal soul is that part of you which thinks and feels and acts when you yourself have ceased to think or feel or act. Of the same order of absurdity is the statement that the Cheshire Cat's grin remained after the Cat had disappeared. Some such absurdities are very easy to recognise, and are funny; and others are not so easy to recognise, and are serious. To the latter class belong those absurdities which result from breach of the general principles of materialism and dialectics. The point of these general principles, and what makes their formulation worthy of being called "the science of thought and its laws", and their observance as essential for right thinking as is observance of the laws of formal logic, is that they are the principles for maintaining informativeness and removing illusion and fantasy.
We can very well speak, if we want to, in violation of laws of dialectics, of things which are not affected by other things and are eternal, indestructible and changeless. Language permits us so to speak, and it involves no grammatical or syntactical error, no violation of principles of formal logic and no logical contradiction. But if we try to describe a world consisting of such things, then we are describing something unrecognisable and unverifiable. For how could we recognise anything or verify its existence unless it affected us in some way? And how can anything which is fixed and changeless and does not interact with anything else possibly so affect us? To speak of such things is to speak of things in principle unverifiablelike, for example, Leibniz's monads. Statements about them are uninformative or, as Engels put it, "fantastic", or as Ryle puts it, “absurd".
Similarly with quantitative specifications unconnected with qualitative ones, or with processes free of opposition or dialectical contradiction. For example, we can very well describe a physical system in which the law of gravity did not hold in the form we know it. Thus bodies might attract each other with a force not inversely proportional to the square of their distance; or in the formulation made by general relativity theory, quite different geometries might apply. But what of a physical system in which forces acted between bodies but none of these forces were attractive? It is not very hard to see that such a system is an "absurdity". One can talk of such a system, but it is mere "fantasy". So in Dialectics of Nature Engels wrote about the necessary correlation of attraction and repulsion in physical systems, and showed that this was a case of the more general principle of "unity of opposites". Again, one can speak of a system in which quantitative changes did not involve qualitative ones. But how could we recognise or measure any quantitative changes unless there were recognisable qualities connected with them? And how could we formulate generalisations or "laws" governing such changes except in terms of such connection? As Engels put it in Dialectics of Nature, "the dialectical laws are really laws of development of nature, and are valid for theoretical natural science". They are "laws of development of nature", and theoretically "valid", because no material system and its laws could be described in concrete terms except in conformity with the dialectical principles. Hence they are always and necessarily found exemplified in natural as well as social science, whenever we undertake "the concrete analysis of concrete conditions". Their formulation is first suggested as a result of observing their occurrences and exemplification (somewhat as mathematics is first arrived at from observation and practical application, and not produced ready‑made a priori out of people's heads, the concepts of number and measure, just like tho