unification of many different concepts, entities and phenomena into an ever smaller and more fundamental number of them. However, it was only in later times that we realized what we were doing and started doing it consciously, setting explicitly the unification of all forces and particles as our major goal.

It is this (sometimes feverish) pursuit of unification that has lead us to the vacuum selection problem in Superstring Theory and similar unification schemes that include gravity. If unification is a major goal, then, the vacuum selection problem is a major problem of Superstring Theory, perhaps the most important one.

In order to get some perspective over this problem we are going to review several instances of unification in Physics. We could go back to Archimedes or Newton but we will content ourselves with the *classical* period of unification that starts with Faraday and Maxwell, showing also that the process of unification underlies all the main advances in Theoretical Physics and is, in particular, strongly related to the symmetry principles on which many of our theories are based.

1.  ${\rm \bf Electricity}\bigoplus{\rm \bf Magnetism}
    \stackrel{{\rm \bf Faraday,
          Maxwell}}{\Longrightarrow} {\rm \bf Electromagnetism}$

    $$\vec{E},  \vec{B} \,\, \longrightarrow \,\, (F_{\mu\nu})\equiv
    \left(
      \begin{array}{c|c}
    0 