  Dirac sought an interpretation of mathematical formalism in terms of physical
entities and Einstein insisted that physics should describe "the real states of
the real systems". While Bell inequalities put into question the reality of
states, modern device-independent approaches do away with the idea of entities:
physical theory may contain no physical systems. Focusing on the correlations
between operationally defined inputs and outputs, device-independent methods
promote a view more distant from the conventional one than Einstein's
'principle theories' were from 'constructive theories'. On the examples of
indefinite causal orders and almost quantum correlations, we ask a puzzling
question: if physical theory is not about systems, then what is it about? The
answer given by the device-independent models is that physics is about
languages. In moving away from the information-theoretic reconstructions of
quantum theory, this answer marks a new conceptual development in the
foundations of physics.
