  In this white paper we introduce the IMAGINE Consortium and its scientific
background, goals and structure. Our purpose is to coordinate and facilitate
the efforts of a diverse group of researchers in the broad areas of the
interstellar medium, Galactic magnetic fields and cosmic rays, and our goal is
to develop more comprehensive insights into the structures and roles of
interstellar magnetic fields and their interactions with cosmic rays. To
achieve a higher level of self-consistency, depth and rigour can only be
achieved by the coordinated efforts of experts in diverse areas of astrophysics
involved in observational, theoretical and numerical work. We present our view
of the present status of this topic, identify its key unsolved problems and
suggest a strategy that will underpin our work. The backbone of the consortium
is the Interstellar MAGnetic field INference Engine, a publicly available
Bayesian platform that employs robust statistical methods to explore the
multi-dimensional likelihood space using any number of modular inputs. It
provides an interpretation and modelling framework that has the power and
flexibility to include a variety of observational, theoretical and numerical
lines of evidence into a self-consistent and comprehensive picture of the
thermal and non-thermal interstellar media. An important innovation is that a
consistent understanding of the phenomena that are directly or indirectly
influenced by the Galactic magnetic field, such as the deflection of ultra-high
energy cosmic rays or extragalactic backgrounds, is made an integral part of
the modelling. The IMAGINE Consortium, which is informal by nature and open to
new participants, hereby presents a methodological framework for the modelling
and understanding of Galactic magnetic fields that is available to all
communities whose research relies on a state-of-the-art solution to this
problem. (Abridged.)
