The development of statistical methods for valid and efficient probabilistic
inference without prior distributions has a long history. Fisher's fiducial
inference is perhaps the most famous of these attempts. We argue that, despite
its seemingly prior-free formulation, fiducial and its various extensions are
not prior-free and, therefore, do not meet the requirements for prior-free
probabilistic inference. In contrast, the inferential model (IM) framework is
genuinely prior-free and is shown to be a promising new method for generating
both valid and efficient probabilistic inference. With a brief introduction to
the two fundamental principles, namely, the validity and efficiency principles,
the three-step construction of the basic IM framework is discussed in the
context of the validity principle. Efficient IM methods, based on conditioning
and marginalization are illustrated with two benchmark examples, namely, the
bivariate normal with unknown correlation coefficient and the Behrens--Fisher
problem.Comment: 14 pages, 1 figure; to appear in WIREs Computational Statistic