  The general view is that all fundamental physical laws should be formulated
within the framework given by quantum mechanics (QM). In a sense, QM therefore
has the character of a metaphysical theory. Consequently, if it is possible to
derive QM from more basic principles, these principles should be of general,
philosophical nature. Here, we derive the formalism of QM from well-motivated
epistemic principles. A key assumption is that a physical theory that relies on
entities or distinctions that are unknowable in principle gives rise to wrong
predictions. First, an epistemic formalism is developed, using concepts like
knowledge and potential knowledge, identifying a physical state $S$ with the
potential knowledge of the physical world. It is demonstrated that QM emerges
from this formalism. However, Hilbert spaces, wave functions and probabilities
are defined in certain well-defined observational contexts only. This means
that the epistemic formalism is broader than QM. In the fundamental layer of
description, the physical state $S$ is a subset of a state space
$\mathcal{S}=\{Z\}$, such that $S$ always contains many elements $Z$. These
elements correspond to unattainable states of complete knowledge of the world.
The evolution of $S$ cannot be determined in terms of the individual evolution
of the elements $Z$, unlike the evolution of an ensemble in classical phase
space. The evolution of $S$ is described in terms of sequential time $n\in
\mathbf{\mathbb{N}}$, which is updated according to $n\rightarrow n+1$ each
time an event occurs, each time potential knowledge changes. Sequential time
$n$ can be separated from relational time $t$, which describes distances
between events in space-time. There is an entire space-time associated with
each $n$, in which $t$ represents the knowledge at sequential time $n$ about
the temporal relations between present and past events.
