Here's the "one science" passage from the Economic and Philosophical Manuscripts:
"We see how subjectivity and objectivity, spirituality and materiality, activity and suffering, lose their antithetical character, and – thus their existence as such antitheses only within the framework of society; we see how the resolution of the theoretical antitheses is only possible in a practical way, by virtue of the practical energy of man. Their resolution is therefore by no means merely a problem of understanding, but a real problem of life, which philosophy could not solve precisely because it conceived this problem as merely a theoretical one.
We see how the history of industry and the established objective existence of industry are the open book of man’s essential powers, the perceptibly existing human psychology. Hitherto this was not conceived in its connection with man’s essential being, but only in an external relation of utility, because, moving in the realm of estrangement, people could only think of man’s general mode of being – religion or history in its abstract – general character as politics, art, literature, etc. – as the reality of man’s essential powers and man’s species-activity. We have before us the objectified essential powers of man in the form of sensuous, alien, useful objects, in the form of estrangement, displayed in ordinary material industry (which can be conceived either as a part of that general movement, or that movement can be conceived as a particular part of industry, since all human activity hitherto has been labour – that is, industry – activity estranged from itself.)
A psychology for which this book, the part of history existing in the most perceptible and accessible form, remains a closed book, cannot become a genuine, comprehensive and real science. What indeed are we to think of a science which airily abstracts from this large part of human labour and which fails to feel its own incompleteness, while such a wealth of human endeavour, unfolded before it, means nothing more to it than, perhaps, what can be expressed in one word – “need”, “vulgar need”?
The natural sciences have developed an enormous activity and have accumulated an ever-growing mass of material. Philosophy, however, has remained just as alien to them as they remain to philosophy. Their momentary unity was only a chimerical illusion. The will was there, but the power was lacking. Historiography itself pays regard to natural science only occasionally, as a factor of enlightenment, utility, and of some special great discoveries. But natural science has invaded and transformed human life all the more practically through the medium of industry; and has prepared human emancipation, although its immediate effect had to be the furthering of the dehumanisation of man. Industry is the actual, historical relationship of nature, and therefore of natural science, to man. If, therefore, industry is conceived as the exoteric revelation of man’s essential powers, we also gain an understanding of the human essence of nature or the natural essence of man. In consequence, natural science will lose its abstractly material – or rather, its idealistic – tendency, and will become the basis of human science, as it has already become – albeit in an estranged form – the basis of actual human life, and to assume one basis for life and a different basis for science is as a matter of course a lie. The nature which develops in human history – the genesis of human society – is man’s real nature; hence nature as it develops through industry, even though in an estranged form, is true anthropological nature.
Sense-perception (see Feuerbach) must be the basis of all science. Only when it proceeds from sense-perception in the two-fold form of sensuous consciousness and sensuous need – is it true science. All history is the history of preparing and developing “man” to become the object of sensuous consciousness, and turning the requirements of “man as man” into his needs. History itself is a real part of natural history of nature developing into man. Natural science will in time incorporate into itself the science of man, just as the science of man will incorporate into itself natural science: there will be one science.
Man is the immediate object of natural science; for immediate, sensuous nature for man is, immediately, human sensuousness (the expressions are identical) – presented immediately in the form of the other man sensuously present for him. Indeed, his own sense-perception first exists as human sensuousness for himself through the other man. But nature is the immediate object of the science of man: the first – object of man – man – is nature, sensuousness; and the particular human sensuous essential powers can only find their self-understanding in the science of the natural world in general, just as they can find their objective realisation only in natural objects. The element of thought itself – the element of thought’s living expression – language – is of a sensuous nature. The social reality of nature, and human natural science, or the natural science of man, are identical terms."
Notice that, contra Lukács, for Marx "nature as it develops through industry, even though in an estranged form, is true anthropological nature" - no 1st and 2nd nature distinction here, or the distinction is one within human nature.
The Richard Gunn articles where he argues against a theory/meta-theory distinction in Marxian theoretical practice are:
Gunn, R. (1987a), ‘Marxism and Philosophy’, Capital & Class 37.
Gunn, R., "Marxism, Metatheory and Critique", http://www.riff-raff.se/en/furtherreading/gunn_metatheory.php
Gunn, R. (1992), `Against Historical Materialism: Marxism as First-Order Discourse', in W. Bonefeld, R. Gunn, K Psychopedis (eds) Open Marxism, Vol. II, Theory and Practice, (London: Pluto) pp. 1-45.