The two āanimalā souls ā intellect and speech:[1]
It states in scripture that one should serve G-d with all of oneās soul [i.e. Bechol Nafshicha]. This refers to the two [non-G-dly] souls that one contains, one being the intellectual soul [i.e. Nefesh Hasichlis] and the second being the speaking soul [i.e. Nefesh Hamidaberes]. Both souls are made up of letters of speech, however, the first is made up of letters of thought while the second is made up of letters of speech. One is instructed to serve G-d with both of the souls. This is accomplished through deeply contemplating with all of oneās mind subjects of Torah, to the maximum that his intellect can reach in order so his entire intellect be fully attached to G-d. In addition, he must also verbalize the words of Torah in order to draw down the energy of the supernal thought from concealment to a revealed state.
[1] Likkutei Torah p. 42a