We all know callus as the thickening the skin in response to repeated friction and pressure. Guitarists develop calluses on their fingertips, bowlers on their thumb as well as their middle and ring fingers. Even children playing on the monkey bars develop calluses just above the palms of their hands. But come to think of it, it is our bodies way of protecting itself from further harm.
In life, we develop our inner calluses when we conquer our bad experience. We become more stronger, more mature and definitely wiser. No one is doomed to fail and never stand up from it. Consider the likes of Donald Trump, he did fail and worse everybody knew about it, yet you never did see him weep rather the next day you saw picking up his pace and creating his new business. Another good example is Henry Ford. He was an illiterate at his time yet look at the break through he made in the automotive industry. When forwarded with a yet similar situation with a previous bad experience, let your callus be your proof that you can handle it.
Another callus in our life is when we continue to learn knowledge. Knowledge is endless and boundless. It continues to innovate and to reproduce itself. People who are the so-called "know-it-all" are usually the ones who are victims of their own premonitions. These are the ones who would always be full of themselves rather than seeing opportunities of new learning to uncover. Like callus, when we stop learning our brain begins to decrease its power, it slims down in its initial state. Always remember that as you continue to learn new knowledge your callus thickens which would serve as your protection from problems.