SimchaMinute by Rabbi Avigdor Miller
Tefilla is what a man thinks in his mind... The multitude has exchanged that for externality, and they consider that they have discharged their obligations by saying so many words and making certain motions and they go home satisfied.
But actually tefilla is nothing but one’s thoughts... As much as you think, that’s how much prayer you’re praying... Like the Rambam says, "Kol tefilla b’lo kavana aino tefilla — Any tefilla without thought is not called tefilla." — Awake My Glory (#88)