Soren Kierkegaard (Danish philosopher) is widely considered the father of existential philosophy. He came up with two concepts that are commonplace to us today:
1- "subjectivity" the idea that we all perceive the world - and "truth" - differently
2- The fact that "faith" is not possible without doubt. One must doubt the existence of God to have faith in the existence of God. Belief without doubt is just credulity.
From his Quotes:
The truth is a trap: you cannot get it without it getting you; you cannot get the truth by capturing it, only by its capturing you.
It is quite true what philosophy says; that life must be understood backwards. But then one forgets the other principle: that it must be lived forwards.
- A book is a door, you know. Always and forever. A book is a door into another place and another heart and another world.
- Readers will always insist on adventures, and though you can have grief without adventures, you cannot have adventures without grief.
No matter what you write, you actually can't help retelling a fairy tale somewhere along the way.
~ Catherynne M. Valente