e has reached exactly the
same conclusion.

Our fundamental conceptions of the external world are therefore derived
from and are built up out of the data of our exertional Activity
combined with the interruptions which that Activity perpetually
encounters, and in which sensations arise. It would indeed be a useful
work of psychological analysis if the conditions of exertional action
were carefully and systematically investigated--much more useful than
most of the trifling experiments to which psychological laboratories are
usually devoted.

The principal elements of such a scheme would be--

(1) The force of gravity. This force constantly operating constrains
the organism to be in constant contact with the earth on which we live.
But, further, it gives us the definite idea of _Direction_. It is from
the action of gravity that we derive our distinction between Up and Down
from which as a starting-point we build up our conception of
tridimensional Space. And in this respect it must be remembered that as
the areas of spheres are proportional to the squares of their radii it
necessarily follows that gravity if it acts uniformly in tridimensional
Space _must_ vary in intensity in proportion to the square of the
distance of the point of application from the centre of origin. Gravity
and tridimensionality are in short necessarily connected.

(2) The same law which determines the force of gravity seems to
determine also the force of cohesion, and therefore the form of material
bodies. These, therefore, are necessarily subject also to
tridimensionality, and in the force which generates solid form we find a
second source of our elementary spatial ideas.

Such form is the expression of an obstacle to action which determines
all our movements, and in which we discover those forms of the
limitations of activity which we call spatial characters.

(3) Organic Dualism is a third determinant of activity, and thus also a
source of spatial ideas.

The structural dualism of the human body, its right and left, its front
and back, etc., furnish our activity with a set of constant forms to
which its action must conform, and which necessarily also partake of,
and help us to conceive of tridimensional form. It is interesting to
note that this dualism characterises the organs specially adapted to
serve exertional action rather than those which serve our vegetal or
nutrient life.

The way in which our spatial conceptions are ever extend