Broken down foundationalism makes two claims.
(1) Epistemic Independence Requirement (EIR). There must be cognitive states that are basic in the sense that they possess some positive epistemic status independently of their epistemic relations to any other cognitive states.
(2) Epistemic Efficacy Requirement (EER). Every none-basic cognitive state can possess positive epistemic status only because of the epistemic relations it bears, directly or indirectly, to basic cognitive states. Thus the basic states must provide the ultimate support for the rest of our knowledge. Such basic, independent and efficacious, cognitive states would be the given.
Sellar argues that you cannot have both. The standard candidates for basic empirical knowledge either fail EER (e.g., knowledge of sense-data), or presuppose other knowledge on the part of the knower, and thus fail EIR (e.g., knowledge of appearances).