Marcus Aurelius: “The happiness of your life depends on the quality of your thoughts.”
Rene Descartes: “I think, therefore I am.”
Antoine Arnauld: “Men are everywhere confronted with alternative routes–some true and others false–and reason must choose between them. Who chooses well has a sound mind, who chooses ill a defective one. Capacity for discerning the truth is the most important measure of minds.”
Mary Wollstonecraft: “The neglected education of my fellow-creatures is the grand source of the misery I deplore.”
John Stuart Mill: “Actions are right in proportion as they promote happiness, wrong as they produce the reverse.”
Martin Heidegger: It is only in full awareness of our own mortality that life can take on any purposive meaning. (summarized)
Jean-Paul Sartre: It is up to the individual to choose the life s/he thinks best. (summarized)
Kurt Godel: The human mind is capable of working out truths that no mechanical procedure can decide. (summarized)