Marx writes to his publisher-friend to explain, clearly, why it is a critique of political economy:
And then the vulgar economist thinks a great discovery has been made when, as against the revelation of the inner interconnection of value and things, the proud claim is that in appearance things look different. In fact, the boast claims to hold fast to appearance, and takes it for the ultimate. Why, then, have any science at all?
But the matter has also another background. Once the interconnection is grasped, all theoretical belief in the permanent necessity of existing conditions collapses before their collapse in practice. Here, therefore, it is absolutely in the interest of the ruling classes to perpetuate a senseless confusion. And for what other purpose are the sycophantic babblers paid, who have no other scientific trump to play save that in political economy one should not think at all?
But satis superque [there you go, and then some]. In any case it shows what these priests of the bourgeoisie have come down to, when workers and even manufacturers and merchants understand my book [Capital] and find their way about in it, while these “learned scribes” (!) complain that I make excessive demands on their understanding….
From 11 July 1868 letter of Marx to Kugelman. Marx and Engels, Selected Correspondence, Moscow, 1955, pp. 250-53. The bold and the italics (except the Latin) are mine, to highlight the choice slurs, and the consequences.
Edit: Lenin says of this: ‘ It is only to be hoped that every one who begins to study Marx and read Capital will read and re-read this letter when studying the first and most difficult chapters of that book’ (Lenin: Published in 1907 in the pamphlet: Karl Marx. Letters to Dr. Kugelmann, edited and with a preface by N. Lenin. Novaya Duma Publishers, St. Petersburg. Published according to the text of the pamphlet.
Source: Lenin Collected Works, Foreign Languages Publishing House, 1962, Moscow, Volume 12, pages 104-112. https://www.marxists.org/archive/lenin/works/1907/feb/05.htm#bkV12E034)