What philosophical theory changed your thinking in life?

For me personally, the philosophical theory of Existentialism put forward by Jean-Paul Sartre has changed many of my views, faiths, and principles about life — especially about being a human being who lives life. What strikes my mind is actually a simple analogy to the concept of Existentialism, which can be said to be the gateway to Sartre's more complex teachings. In his theory he compares a pair of scissors with a human, where the difference between the two is based on 2 values, namely: Existence and Essence. Existence represents the form, appearance, idea, specification, or natural properties attached to an object.

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While the Essence represents the function, benefit, or definition that determines the identity of an object. Scissors, in the process of its birth, was initiated by an idea in the human head about a tool that could cut paper, or rope, or other objects practically. In that idea also arose the need for how these scissors could be controlled by hand, even with just 2 fingers.

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So the concept of scissors has been defined, its function has been defined, and its form has been described even before the name "scissors" appeared. Here it means "essence precedes existence", where the function is first determined and then the form appears. Whereas humans have the opposite process. Humans cannot be determined by their function and value when they are born. No one can determine that a baby should become a doctor, poet, philosopher, or anything other than the baby itself. Here "existence precedes essence" — meaning that every human being has full responsibility to determine their value, benefit, or function in life.

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So for me the task of every human being is to continue to define their essence either in deed or thought in every day that passes. At first glance it may sound like free will, but behind it lies the restlessness and need of humans to actualize themselves positively for life.

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Sartre's theory enlightened me that first, philosophy is important, and second, life is far more important. He whispered to me that in this life it is not enough for humans to just "experience" but also be able to "be".