Socrates once said (at least that's what they say he said) "I think therefore I am" meaning the very essence of being human is the ability to process thoughts. There are a billion things that could run through a person's mind each day. Sometimes that ability can push one to a state of mental exhaustion that weakens the body leaving it hollow. Hence, one at such moments can say "I think therefore I am tired". Lethargy was created by applying a mosaics effect on an original digital painting.