took to write it ourselves, we soon became aware of the
difficulties involved. We had no models to work from, no
tradition to follow, at the undergraduate level of exposition.
With respect to the content as well as the form of our text,
we had to rely upon our own judgment, again and again --
often with misgivings.

We found, too, that several readers had to be kept before
us as we wrote. There was the beginning student, the object
of our greatest concern, who might never go beyond an intro-
ductory course. There was the advanced undergraduate, even
the graduate student, who had not yet been acquainted with
our way of thinking. And, finally, there were our colleagues,
watching us to see how well we would be able to carry rein-
forcement theory to the student.

Now, with our book in the hands of the publisher, we are
well aware that it reflects our struggle with these problems,
rather than their complete solution. No one could think
otherwise. We will have to be content, at present, with some-
thing less than the loaf we intended to put on your table. On
the other hand, we think there is something nutritious in it.
How well it has been kneaded, and how well baked, the
reader must judge.

All books are indebted to more people, for more things,
than the authors can remember. At this juncture', we recall
most clearly Richard M. Elliott, whose confidence in our
project never faltered and who was most generous with his
reinforcements; Thomas W. Reese and Kenneth MacCor-
quodale who, with gentle painstaking, rooted out errors and
added their thoughts to our own at many points; and Murray
Sidman and James A. Dinsmoor who, with great kindness,
took up the double burden of illustration and indexing. More
than by any other man, we have been guided and inspired by
the work and thought of Burrhus F. Skinner, who led us into
the field of behavior theory, and whose influence has so per-
fused this field as to be lost to a proper contemporary perspec-

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