  There are (at least) four ways that an agent can acquire information
concerning the state of the universe: via observation, control, prediction, or
via retrodiction, i.e., memory. Each of these four ways of acquiring
information seems to rely on a different kind of physical device (resp., an
observation device, a control device, etc.). However it turns out that certain
mathematical structure is common to those four types of device. Any device that
possesses a certain subset of that structure is known as an "inference device"
(ID).
  Here I review some of the properties of IDs, including their relation with
Turing machines, and (more loosely) quantum mechanics. I also review the bounds
of the joint abilities of any set of IDs to know facts about the physical
universe that contains them. These bounds constrain the possible properties of
any universe that contains agents who can acquire information concerning that
universe.
  I then extend this previous work on IDs, by adding to the definition of IDs
some of the other mathematical structure that is common to the four ways of
acquiring information about the universe but is not captured in the (minimal)
definition of IDs. I discuss these extensions of IDs in the context of
epistemic logic (especially possible worlds formalisms like Kripke structures
and Aumann structures). In particular, I show that these extensions of IDs are
not subject to the problem of logical omniscience that plagues many previously
studied forms of epistemic logic.
