There is a theory of truth called 'correspondence theory of truth'. It says that x is true if and only if x corresponds to the fact. However, many philosophers have objected to correspondence theory of truth for many reasons. Alternative positions are deflationary theory of truth, coherence theory of truth, and so on.

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'Science and Metaphysics' is a good starting point to examine Sellars' philosophy of perception. It would be helpful to understand 'Empiricism and the Philosophy of Mind'.

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Is it a general expression?

According to Sellars, sense-data are not epistemic elements. Empiricists thought that sense-data are directly 'given' to epistemic subjects. Sellars thought that sense-data should be constituted by linguistic ways to be epistemic. This is called Sellars' psychological nominalism. According to psychological nominalism, language is prior to sense-data.

A different approach is that 'I think, therefore I am' means 'my mind works by thinking'. Descartes is regarded as one of the ancestors of mental representation theorists. Descartes thought that persons are the same as souls. He thought that souls operate by thinking. According to Descartes, the essence of mind is (mental) thought.
Similar arguments appeared in Kripke's argument against materialism. According to Kripke, Descartes is not identical to Descartes' body. When Descartes is dead, the corpse of Descartes is not identical to him.
Descartes did several things by 'Cogito, ergo sum': He derived his existence from the statement. He defined the nature of 'self' or 'soul' or 'mind'.

Empiricism and the Philosophy of Mind is the most famous article written by Wilfrid Sellars. Harvard Edition includes Robert Brandom's study guide. Robert Brandom's Articulating Reasons will help you understand Sellars' thoughts.