"That’s why the philosophers warn us not to be satisfied with mere learning, but to add practice and then training. For as time passes we forget what we learned and end up doing the opposite, and hold opinions the opposite of what we should." -Epictetus (via The Daily Stoic)
"The person who has practiced philosophy as a cure for the self becomes great of soul, filled with confidence, invincible—and greater as you draw near." -Seneca (via The Daily Stoic)
The above quotes reminded me of Charlie Munger's Use-It-or-Lose-It Tendency from his "The Psychology of Human Misjudgment" speech. Why take the time to develop and curate a philosophy — on life, investing, diet/health, etc. — if one isn't going to practice it on a constant basis? As Munger wrote in his speech:
All skills attenuate with disuse. I was a whiz at calculus until age twenty, after which the skill was soon obliterated by total nonuse. The right antidote to such a loss is to make use of the functional equivalent of the aircraft simulator employed in pilot training. This allows a pilot to continuously practice all of the rarely used skills that he can’t afford to lose.
Throughout his life, a wise man engages in practice of all his useful, rarely used skills, many of them outside his discipline, as a sort of duty to his better self. If he reduces the number of skills he practices and, therefore, the number of skills he retains, he will naturally drift into error from man with a hammer tendency. His learning capacity will also shrink as he creates gaps in the latticework of theory he needs as a framework for understanding new experience. It is also essential for a thinking man to assemble his skills into a checklist that he routinely uses. Any other mode of operation will cause him to miss much that is important.
Skills of a very high order can be maintained only with daily practice. The pianist Paderewski once said that if he failed to practice for a single day, he could notice his performance deterioration and that, after a week’s gap in practice, the audience could notice it as well.
The hard rule of Use-It-or-Lose-It Tendency tempers its harshness for the diligent. If a skill is raised to fluency, instead of merely being crammed in briefly to enable one to pass some test, then the skill (1) will be lost more slowly and (2) will come back faster when refreshed with new learning. These are not minor advantages, and a wise man engaged in learning some important skill will not stop until he is really fluent in it.
And specifically in regards to investing, Tren Griffin added this in his book on Charlie Munger:
To be a successful investor, a person must regularly devote the necessary time and effort. Even if you once felt that you knew a lot about investing, it does not mean your skills are current. Maintaining a circle of competence requires constant work and diligence.