Friedrich Engels, answered from the texts and cited to the page.
The laws are three, and they are not my inventions — they are abstractions from the actual history of nature and human society, turned right-side up from the idealist mystification Hegel left them in.1 The first is the law of the transformation of quantity into quality and vice versa. Qualitative changes in nature occur only through the quantitative addition or subtraction of matter or motion — what we now call energy.2
Water does not gradually become ice; at a precise quantity of heat removed, the qualitative leap occurs. The principle is not mysterious once you strip away the Hegelian fog; it is, as I wrote in Anti-Dühring, rather obvious.3 The second is the law of the interpenetration of opposites. Consider motion itself: simple mechanical displacement from place to place can only occur by a body being simultaneously in one place and in another — present and not present at the same point.4
Contradiction is not an absurdity confined to thought, as Dühring insisted; it is the actual structure of every moving, changing, living thing. Regard things as static, isolated, and you find no contradiction; follow them in their movement and mutual relations, and contradiction is everywhere.5 The third is the law of the negation of the negation.
This is the one most commonly caricatured. Negation in dialectics does not mean simply saying no, or destroying something in any way one likes — grinding a grain of barley negates it, but makes the second negation impossible, and you get no barley plant, only flour.6 The dialectical negation must be set up in such a way that the second negation, the negation of the negation, remains possible; and the specific form that takes is determined by the particular nature of each individual case.7
The grain becomes a plant becomes many grains — that is development through negation, not mere cancellation. All three were developed by Hegel, but in idealist fashion, as mere laws of thought — foisted on nature rather than deduced from it.8 That is the whole source of the forced and often outrageous treatment in his system, where the universe is made to conform to a stage of human thinking rather than the thinking being recognized as a product of nature's own development. Turn the thing round, and what looked mysterious becomes, as I put it, simple and clear as noonday.9
It is, therefore, from the history of nature and human society that the laws of dialectics are abstracted. For they are nothing but the most general laws of these two aspects of historical development, as well as of thought itself.Dialectics of Nature, pp. 61–63
in nature, in a manner exactly fixed for each individual case, qualitative changes can only occur by the quantitative addition or quantitative subtraction of matter or motion (so-called energy).Dialectics of Nature, pp. 63–64
In this form, therefore, Hegel's mysterious principle appears not only quite rational but even rather obvious.Dialectics of Nature, pp. 63–64
Motion is itself a contradiction since simple mechanical movement from place to place can only accomplish itself by a body being at one and the same moment in one place and simultaneously in another place by being in one and the same place and yet not there.Landmarks of Scientific Socialism - Anti-Duhring
As long as we regard things as static and without life, each by itself, separately, we do not run against any contradictions in them... But it is quite different when we consider things in their movement, in their change, their life and their mutually reciprocal relations. Then we come at once upon contradictions.Landmarks of Scientific Socialism - Anti-Duhring
If I grind a grain of barley, if I crush an insect, it is true I have carried out the first act, but have made the second act impossible.Anti-Dühring, pp. 152–153
I must therefore set up the first negation in such a way that the second remains or becomes possible... every kind of thing has its characteristic kind of way of being negated, of being negated in such a way that it gives rise to a development.Anti-Dühring, pp. 152–153
All three are developed by Hegel in his idealist fashion as mere laws of thought... The mistake lies in the fact that these laws are foisted on nature and history as laws of thought, and not deduced from them.Dialectics of Nature, pp. 61–63
If we turn the thing round, then everything becomes simple, and the dialectical laws that look so extremely mysterious in idealist philosophy at once become simple and clear as noonday.Dialectics of Nature, pp. 63–64