Conceptual change and its connection to the development of new seien-
tific theories has reeently beeome an intensively discussed topic in
philo- sophieal literature. Even if the inductive aspects related to
conceptual change have already been discussed to some extent, there has
so far existed no systematic treatment of inductive change due to
conceptual enrichment. This is what we attempt to accomplish in this
work, al- though most of our technical results are restricted to the
framework of monadic languages. We extend Hintikka's system of inductive
logic to apply to situations in which new concepts are introduced to the
original language. By interpreting them as theoretica1 concepts, it is
possible to discuss a number of currently debated philosophical and
methodological problems which have previously escaped systematic and
exact treatment. For instance, the role which seientific theories
employing theoretical con- cepts may play within inductive inference can
be studied within this framework. From the viewpoint of seientific
realism, sueh a study gives outlines for a theory of what we call
hypothetico-induetive inference. Some parts of this work which are based
on Hintikka's system of in- ductive logic are fairly technical. However,
no previous knowledge of this system is required, but, in general,
acquaintance with the basic ideas of elementary logic and probability
theory is suffieient. This work is part of a project, originated by
Professors Jaakko Hintikka and Raimo Tuomela, concerning the role of
theoretical concepts in science.