Perhaps the most famous quote in all of philosophy, "I think, therefore I am" comes from René Descartes' 1637 work, Discourse on the Method. It is often misattributed to Descartes' later work Meditations on First Philosophy but does not appear verbatim in that text. The point of the quote is that, even though Descartes could doubt the existence of everything - his body, the physical world, and even mathematical statements like "2+2=4" - he can't doubt that he exists. Descartes' reasoning is that, since thinking requires a thinker, the very fact he is able to think about and doubt his existence proves he does exist. Hence, "I think, therefore I am".
"It is better to be a human being dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied."
From Mill's 1861 essay Utilitarianism, this quote reflects his qualitative distinction between 'higher' and 'lower' pleasures. As a (hedonistic) utilitarian, Mill believed that what makes an action good or bad is the consequences that result from it - actions that increase pleasure are good, actions that decrease pleasure or increase pain are bad. But Mill rejected Bentham's purely quantitative approach as too simplistic and a 'doctrine of swine'. For Mill, moral value does not come from simply the *amount* of pleasure that results, but also the *kind* of pleasure. Mill says that some pleasures - such as those of thought and morality - are better and more morally valuable than the pleasures that can also be felt by animals, such as eating and sex.
"wherein men live without other security, than what their own strength, and their own invention shall furnish them withall [...] there is [...] continuall feare, and danger of violent death; And the life of man, solitary, poore, nasty, brutish, and short."
This quote is from Thomas Hobbes' 1651 work Leviathan. Hobbes is describing human life in a 'state of nature' where there is no central government. Because humans are naturally competitive and self-interested, he argued, we need a social contract and a strong government to enforce it in order to avoid a war of all against all.
"One swallow does not make a summer; neither does one day. Similarly neither can one day, or a brief space of time, make a man blessed and happy."
From his Nicomachean Ethics (written around 330 BC), the point of the quote is that eudaimonia (which means human flourishing, happiness, a good life - it's hard to translate exactly) is not a fleeting emotion or a temporary state you can gain or lose from one day to the next. Instead, eudaimonia is the product of a lifetime of virtuous action. Just as a single warm day or the arrival of one bird doesn't mean the season has changed, Aristotle argues that true happiness requires a life of consistent, habitual excellence rather than a few isolated good moments.
"existence precedes essence"
From Sartre's 1946 essay Existentialism Is a Humanism, Sartre's point is that human beings find themselves existing in the universe without a purpose, and so humans must define their own meaning and "essence" through their individual choices and actions - that existence comes first, then meaning second. This contrasts with, for example, the Aristotelian view that humans have an inherent telos or purpose (to act according to reason).
"Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas:—How comes it to be furnished? [...] To this I answer, in one word, from EXPERIENCE."
This quote is from John Locke's 1690 work An Essay Concerning Human Understanding. Locke is often said to have described the mind at birth as a "blank slate" or "tabula rasa" - but that quote never actually appears verbatim in Essay Concerning Human Understanding. Instead, in his arguments against innate knowledge, Locke describes the mind at birth as a sheet of "white paper" on which experience writes.
"Two things fill the mind with ever new and increasing admiration and awe, [...] the starry heavens above and the moral law within."
This quote is the conclusion to Kant’s 1788 work the Critique of Practical Reason. It bridges two great themes from his works: (1) the vast, orderly universe revealed by science ("the starry heavens above") and (2) the inner capacity of human reason to recognise moral duty ("the moral law within"). For Kant, morality does not come from external authority but from rational beings legislating universal moral laws for themselves (the categorical imperative).
"He who fights with monsters should be careful lest he thereby become a monster. And if thou gaze long into an abyss, the abyss will also gaze into thee."
From Nietzsche's 1886 work, Beyond Good and Evil, this aphorism warns that in confronting the inherent meaninglessness (the abyss) of the world, one risks being corrupted or consumed by the very darkness they are investigating.
"If a lion could talk, we could not understand him."
From Wittgenstein's 1953 Philosophical Investigations, this quote illustrates Wittgenstein's view that language and the meaning of words is inextricably tied to how they are used within a 'form of life'. The point is that because a lion's life, instincts, and daily experiences are so fundamentally different from those of a human, we would lack the shared context and 'language-games' necessary to actually comprehend what the lion meant - even if it could produce speech.
"It appears, then, that this idea of a necessary connexion among events arises from a number of similar instances which occur of the constant conjunction of these events; [...] But there is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar; except only, that after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist."
From Hume's 1748 work, An Enquiry Concerning Human Understanding, this is his argument that our belief in causality - what he calls 'necessary connexion' here - is not justified by reason or observation. Instead, it is a habit of the mind formed after seeing one event constantly follow another - we just expect the future to resemble the past out of custom. We see, for example, several instances of a snooker ball moving when struck by another snooker ball and then assume the first snooker ball caused the second snooker ball to move. But this belief of causation/necessary connexion is not justified by reason or observation.
"Act only according to that maxim whereby you can at the same time will that it should become a universal law."
From his 1785 work Groundwork of the Metaphysics of Morals, this is Kant's 'categorical imperative' - the ultimate test for whether an action is morally acceptable or not. The point of the quote is that before you act you should ask yourself: "would I be okay if every single person on Earth always followed the rule of how I'm about to act?" If the answer is no, says Kant, then that action is morally wrong.
"Their ESSE is PERCIPI, nor is it possible they should have any existence out of the minds or thinking things which perceive them."
This famous quote is the central thesis of Berkeley’s 1710 work, A Treatise Concerning the Principles of Human Knowledge. Berkeley argues for subjective idealism - the view that what we think of as mind-objects (e.g. tables, chairs, etc.) do not, in reality, exist independently of a mind. Instead, their entire existence consists in their being perceived - to be is to be perceived. In other words, if an object isn't being perceived by someone, then it can't exist.
"If you marry, you will regret it; if you do not marry, you will also regret it; if you marry or do not marry, you will regret both;"
This quote is from the 'Diapsalmata' section of Søren Kierkegaard’s 1843 book, Either/Or. Writing as the character 'A', Kierkegaard argues that human choice inevitably leads to regret because choosing one thing requires giving up another. He concludes that because earthly choices are limited, people must look to a spiritual level of existence to find meaning.
"the unexamined life is not worth living"
This quote appears in Plato's dialogue Apology (written around 390 BC), which describes the trial of Socrates - Plato's teacher. Within the dialogue, it's actually Socrates who says the quote, but it reflects Plato's view that a life spent without self-reflection and pursuit of truth lacks the essential quality that makes us human.
