Is the idea that nothing can be said to exist beyond the observation of the thing itself. So, for example, you could not argue that the stone exists, only that your sense of it exists. You could say: “I saw a stone.” but not: “The stone was there.” The only thing that one is able to confirm is the sensory data of the stone, but not the stone’s existence independent of your own.

The idea that only the present exists, and that both the past and future do not. A Buddhist scholar named Fyodor Shcherbatskoy said the following: “Everything past is unreal, everything future is unreal, everything imagined, absent, mental . . . is unreal. . .. Ultimately real is only the present moment of physical efficiency.” The belief that our way of experiencing time is it’s true and only nature. So, for a presentist the idea of time travel is ridiculous, as there exists no destination to travel to, where other philosophies and theories might suggest otherwise.

Contrary to presentism, eternalism is the belief that all moments in time, past, present and future are equally real. Some eternalists believe that because of the nature of time, in this case that time exists as a whole, not in separate parts, the existing future already exists in a set and final manner, and therefore we are only capable of experiencing the future, not able to change it in any way, which one could interpret as the existence of fate. Modern scientific theories seemingly support the eternalism over presentism, but with our ever-developing understanding of the universe.

The most well-known form of nihilism, existential nihilism is focused on the assertion that life has no inherent purpose, goal, or intrinsic value. (Intrinsic value is the idea of something having value in and of itself.) Simplified, it’s the belief that life is utterly pointless. The difference between nihilism and hedonism is that pleasure, or joy, is seen as worthless as well, and therefore is often characterized as leading to a feeling of despair. Some modern interpretations of existential nihilism conclude that precisely because your life has no intrinsic value, goals or purpose.

Hedonism is centered around the belief that pleasure is the only thing that has intrinsic value. Basically, a hedonist makes pleasure the ultimate goal of any and all of his actions and choices in life. Hedonism is perhaps the philosophy that is closest to our original instincts, in that it embraces the response of pleasure to things like eating and fornicating wholeheartedly. Instead of bringing morals into the picture, it focuses on feasting on pleasure, a sensory response that probably played a vital part in our survival as a species.

Unlike what seems to be popular belief, stoicism is not about faking not having an emotional response or becoming completely emotionless. It is a philosophy that focuses on training yourself to improve through training and conditioning. From everything to your outlook on life, to knowledge and perhaps especially minimizing your negative emotional responses. Stoics believe that emotions like anger, sadness and frustration are based in your own, fixable faults rather than justified responses to outside influences.

One could perhaps argue that skepticism is the basis for all other philosophies. Because if we didn’t question, if we didn’t ask, then where would the answers be? But philosophical skepticism, unlike methodical skepticism, does not focus on questioning individual statements to validate or invalidate; rather, it questions if there is a possibility for a certainty in any knowledge. And given the constant changes in our understanding of the universe and even what’s directly in front of us, it might not be as “overly skeptical” as you might think.