In Islam, each directive has a form and a spirit. The form is its
outward manifestation and the spirit its inner one. The form is dependent on
external circumstances, whereas the spirit is not and only requires a person's
sincere will. In ordinary circumstances, a directive is required to be carried
out keeping in consideration both the form and the spirit. However, whenever
it is not possible to carry out the directive in its required form (which as
pointed out above is dependent on certain variables), Islam wants the
directive to be carried out in some reduced or replaced form so that the
spirit part can be kept alive. Consequently, if a person is not able to pray
while standing he has been allowed to pray while sitting or lying down so that
the spirit of the prayer is kept alive.
Tayammum when wudū is not possible should also be understood keeping in
view this principle. It is meant to keep alive the spirit behind the directive
of wudū. Of course it does not clean a person; however, it does make him pure
in his heart if he knows its real philosophy.