"The difference between the stomach and the mind is the stomach will let you know when it’s empty."
I read that quote somewhere years ago, and was struck by the deep truth it reveals. I do not know who said it first, but it is very true. Our physical appetites can overwhelm us, and we feel it when they are in want. Whether it is something we need to survive (like food) or some other unnecessary pleasure, it makes no difference. Our "bodily" cravings will yell at us when they are "running short".
Sometimes those physical desires are good things: like rest, and nourishment. But even those things can be abused until they control us: like sloth, and gluttony. Our minds, however, are much weaker. Because we are often a lazy people, we can easily get lazy with our minds and not even know it. Most people do not like to think about things; they claim to be "free thinkers" but in truth they just want someone to tell them what to do in order satisfy their desire for pleasure.
Thus, when the mind is truly empty it will just slip into laziness and not work to develop greater knowledge and understanding. The big difference is that the stomach needs to be filled for physical survival. The mind needs to be filled for mental survival. The mind is not, however, the same as the spirit, but is rather one aspect of the spirit (along with heart and will). This is why the mind can become "empty" without one knowing it: it can be overshadowed by the heart and will, both of which can deceptively lead us astray.
Although we are both physical and spiritual beings, the physical is easier to recognize (even with little to no intelligence) and the spiritual can easily be missed. We gravitate to thinking about our physical needs, even sometimes to the neglect of our spiritual needs. Therefore: the stomach tells you when it is empty, but the mind does not. And that is why so many people go about satisfying their physical appetites and allow their thinking to become muddy and confused. Pay attention to the needs of your whole spirit: heart, mind, and will.