There are some who are solid and tenacious in their vital, it is they who can be steady – others are more mercurial and easily moved by impulses, it is these who are sometimes enthusiastic, sometimes drop into fatigue. It is a matter of temperament. On the other hand the mercurial people are often capable of a quicker ardour, so that they can progress fast if they want in their own way. In any case the remedy for all that is to find one’s true self above mind and vital and so not bound by temperament.
Ref: Letters on Yoga – IV