_taste_, _smell_, _feel cold_ and _warm_ under certain objective
conditions. In the case of ourselves, we can know both our _actions_ and
our _mind_. In the case of others, we can know only their _actions_, and
must infer their mental states from our own in similar circumstances.
With certain restrictions and precautions this inference is legitimate.

We said the method of psychology is that of observation and experiment.
The experiment is observation still, but observation subjected to exact
methodical procedure. In a psychological experiment we set out to
provide the necessary conditions, eliminating some and supplying others
according to our object. The experiment has certain advantages. It
enables us to isolate the phenomena to be studied, it enables us to vary
the circumstances and conditions to suit our purposes, it enables us to
repeat the observation as often as we like, and it enables us to measure
exactly the factors of the phenomena studied.

=A Psychological Experiment.= Let us illustrate psychological method by a
typical experiment. Suppose we wish to measure the individual
differences among the members of a class with respect to a certain
ability; namely, the muscular speed of the right hand. Psychological
laboratories have delicate apparatus for making such a study. But let
us see how we can do it, roughly at least, without any apparatus. Let
each member of the class take a sheet of paper and a pencil, and make as
many strokes as possible in a half-minute, as shown in Figure I. The
instructor can keep the time with a stop watch, or less accurately with
the second hand of an ordinary watch. Before beginning the experiment,
the instructor should have each student taking the test try it for a
second or two. This is to make sure that all understand what they are to
do. When the instructor is sure that all understand, he should have the
students hold their pencils in readiness above the paper, and at the