Marx fourth lecture
A. The Grundrisse excerpt (pp. 375-84):
1. The first section (pp. 375-7) belongs with "On the Jewish Question," on the individualism of "civil society" (here bourgeois society) as contrasted with true human nature (the human individual as a communal being: "an animal which can achieve individuation only in society"). In bourgeois life "the various forms of association in society appear to the individual simply as means to his private ends, as external necessity" (p. 376). This leads to a mistake about human nature: taking "the individualized individual" to reflect human nature as it timelessly is, rather than what people are like at a certain point in history. Marx finds the mistake in classical political economy and in Rousseau (though he can be seen as building on Rousseau); he might also find it in Hobbes.
2. The rest of the passage attacks a similar mistake by classical economists: taking economic arrangements as they stand at a particular point in history to reflect timeless objective necessities.
B. The Capital excerpt (pp. 444-461), on "the fetishism of commodities":
1. Fetishism is crediting inanimate objects with characteristics that properly belong only to living beings.
2. Proper human self-expression is creative activity in a social context (compare "Alienated Labor"). So properly human relations would be "direct social relations between individuals at work" (p. 447). But in an exchange economy social relations between producers are set up only in the exchange of what they produce (commodities). What should be relations between human beings are relocated in the relations of value between their products that are supposed to underlie the proportions in which the products are exchanged for one another (prices, when exchange is mediated by money). Thus what should be a relation between human beings "assumes … the fantastic form of a relation between things" — fantastic because it's a social relation between things (a case of fetishism as explained under 1 above).
3. Keeping relations between human beings human is ensuring that they are in one respect as they should be, not necessarily in all. They can still be unjust or involve lack of freedom. See p. 453 on the feudal system: it was free of the fantastication of human relations in which they are relocated as relations between things, but marked by "personal dependence."
4. Along with universal exchangeability of products we get a reduction of the different kinds of work "to their common denominator, viz., expenditure of human labour power or human labour in the abstract" (p. 448). The specific character of a worker's work loses meaningfulness; her work becomes just work (to earn wages, to keep her merely individual life going).