TL;DR
Everything you ever felt, both good and bad, went away at some point.
Everything that exists now, both material and immaterial, will end at some point.
While we tend to see the status quo as permanent, in reality, things change all the time — both around and within us.
Although the situations we are in often seem static on a macro-level, circumstances continuously change on a micro-level — day by day. Slightly different things happen around us every day, events pass, serendipity strikes.
Likewise, all of the thoughts we ever thought went away at some point. Even emotions subside eventually, no matter how strongly we feel them.
Panta Rhei, a famous aphorism by Heraclitus, means 'everything flows.' Change is the fundamental force that stoically transforms the past into the present and the present into the future.
The acknowledgement of impermanence is not only a fact-based observation about reality, it is also a core part of Eastern philosophies such as Buddhism and Hinduism, as well as a tenet of ancient Western world views that have impacted thinking for thousands of years.
To Remember
"Nothing endures but change."
— Heraclitus
"No man ever steps in the same river twice."
— Heraclitus
"What makes us suffer is wanting things to be permanent when they are not."
— Thich Nhat Hanh
Explore
➞ For an overview of Impermanence as an important concept in Eastern and Western philosophy, read this Wiki article.
➞ This long and insightful Nautilus article explains how philosophical impermanence can be combined with science.
Resources
For a structured list of fascinating books, blogs, podcasts, and Youtube channels related to cognition and self-perception, visit mindvault.co/vault/impermanence.