Blavatsky
Original sense
The fifth principle, the thinker. Higher Manas is the reincarnating individuality, capable of union with Buddhi. Lower Manas is that same mind snared in Kama, producing the personality — clever, passionate, mortal. The whole human drama is the tug of war between the two.
Manas is dual — the higher, attracted to Buddhi; the lower, attracted to Kâma, the seat of animal desires.
Mentalism in the New Thought / Law of Attraction sense is not her doctrine. Thought is causative, but it is karmically causative over lives, not a vending machine in this one.