Engels discusses this topic in chapter IX of Anti-Dühring (Morality and Law. Eternal Truths). He begins as usual by calling Dühring's statements on this topic BALDERDASH: and well he might since the good Herr begins by saying, "He who can think only by means of language has never yet learnt what is meant by ABSTRACT and PURE thought. "Indeed! Thinking without language? This prompts Engels to say then the "animals are the most abstract and purest thinkers." This quip is reminiscent of Hegel's response to the theologian Schleiermacher who said the essence of Christianity was unquestioning faith in your Lord. Hegel said then "the dog makes the best Christian."
Dühring is not a relativist on the subject of laws and morals. There is only one true moral law, not only for humans but creatures form outer space as well. He says, morals "must occur in concordant fashion among all extra-human beings whose active reason has to deal with the conscious ordering of life impulses in the form of instincts." By "extra-human beings" he means those living "on other celestial bodies." "Rational beings" would be (following Kant) a better term I think.
Dühring is quite insistent about this sort of thing, writing, "GENUINE TRUTHS ARE ABSOLUTELY IMMUTABLE ... so that it is altogether stupid to think that the correctness of knowledge is something that can be affected by time and changes in reality." What he is claiming is that human knowledge can attain, as Engels says, "sovereign validity and an unconditional claim to truth."
Well, is that true? "Is human thought sovereign?" Engels asks us to consider the following (it is very instructive for those who accuse Marxists of being DOGMATIC): "... in all probability we are just at the beginning of human history [not at the End of History as some pundits declared tr], and the generations which will put US right are likely to be far more numerous than those whose knowledge we --- often enough with a considerable degree of contempt --- have the opportunity to correct." This is to ward off Herr Dühring and his absolutely immutable balderdash. With some exceptions Engels has held up fairly well I think (with some refurbishing along the way.)
As for human knowledge being sovereign, Engels says it is "in its disposition, its vocation, its possibilities and its historical ultimate goal; it is not sovereign and it is limited in its individual realization and in reality at any particular moment." As for "eternal truths," Herr Dühring's conception is too idealistic and not of much use in the actual practice of science. Reason would arrive at the point where the intellectual world would be completely at a stand still if we had only Dühring's immutable truths to work with. But this does not mean that there are NO eternal truths at all.
Well there are some such as 2+2=4, water is H2O, Caesar was assassinated in 44 BC. Simple mathematical, chemical, historical truths, etc., but certainly no BIG eternal truths such as Dühring has in mind-- laws of history or of politics.
Especially when we are dealing with social phenomena are we not going to find eternal laws. This type of knowledge is always limited and relative and, as Engels points out , these kinds of law "exist only in a particular epoch and among particular peoples and are by their very nature transitory." And as for the dogmatism of Marxists-- Engels wants to stress that NO "individual whatsoever is in a position to deliver the final and ultimate truth." One can imagine what he would have thought of the Cult of Personality.
Outside of trivial truths we cannot have much faith that absolutely immutable truth is going to be available to us in the physical and social sciences with respect to truth and error, but what about morality and the knowledge of good and evil? Well, throughout history and all over the world we find different moralities and moral outlooks and some "are in direct contradiction to each other."
In the West we have two versions of Christian feudal morality, Catholic and Protestant, and many subdivisions of these as well. No morality is "true" in the sense of ultimate reality. Different classes have different values. There is a bourgeois morality and a working class morality. Engels thinks the morality of the future is, for us, truer than that which represents the past. The working class represents the future of humanity, for Engels, and so as far as "truth" is concerned it is working class morality that is "true" for us. It has been over a hundred and thirty years since Engels wrote Anti-Dühring and we have seen two large scale experiments in working class control-- the Chinese and Soviet experiments. It would be interesting to compare the morality taught in these two dispensations with Western bourgeois notions of morality. The following reference is a place to start [The Role of Morality in Communist Production by GeorgLukács1919 www.marxists.org/archive/lukacs/works/1919/morality.htm. ]
Engels says the three classes of modern society are the feudal aristocracy, the the bourgeoisie and the proletariat. I think by now only the last two have any relevance in the major parts of the world. These two have different moral ideals, although many strata of the proletariat have been contaminated by bourgeois values. But the fact of these two different moral outlooks shows "that men, consciously or unconsciously, derive their ethical ideas in the last resort from the practical relations on which their class position is based--- from the economic relations in which they carry on production and exchange."
If there are areas of agreement between differing moralities, Engels says, this is because they have shared a common historical development and thus overlapping is to be expected. Engels rejects any attempt to impose eternal truths of morality since they are the products of historical conditioning. He also thinks there has been progress in moral ideas as in other fields (science, medicine, industry, etc.) and this is due to the class struggle, the struggle of people to free themselves from exploitation and poverty, which has led to moral reforms. But a truly human morality that rests on foundations independent of class struggle "becomes possible only at a stage of society which has not only overcome class antagonisms but has even forgotten them in practical life."