‘It seems to me what is called for is an exquisite balance between two conflicting needs: the most skeptical scrutiny of all hypotheses that are served up to us and at the same time a great openness to new ideas. Obviously those two modes of thought are in some tension. But if you are able to exercise only one of these modes, whichever one it is, you’re in deep trouble.
If you are only skeptical, then no new ideas make it through to you. You never learn anything new. You become a crotchety old person convinced that nonsense is ruling the world. (There is, of course, much data to support you.) But every now and then, maybe once in a hundred cases, a new idea turns out to be on the mark, valid and wonderful. If you are too much in the habit of being skeptical about everything, you are going to miss or resent it, and either way you will be standing in the way of understanding and progress.
On the other hand, if you are open to the point of gullibility and have not an ounce of skeptical sense in you, then you cannot distinguish the useful as from the worthless ones. If all ideas have equal validity then you are lost, because then, it seems to me, no ideas have any validity at all.
Some ideas are better than others. The machinery for distinguishing them is an essential tool in dealing with the world and especially in dealing with the future. And it is precisely the mix of these two modes of thought that is central to the success of science.’
Read more about Carl Sagan’s view on science. 

We modern people are problem-solvers, but the demand for answers crowds out patience — and perhaps, especially, patience with mystery, with that which we cannot control. Intolerant of ambiguity, we deny our own ambivalences, searching for answers to our most anguished questions in technique, hoping to find an ultimate healing in technology. But feelings of dislocation, isolation, and off-centeredness persist, as they always have.
We are not ‘everything,’ but neither are we ‘nothing.’ Spirituality is discovered in that space between paradox’s extremes, for there we confront our helplessness and powerlessness, our woundedness. In seeking to understand our limitations, we seek not only an easing of our pain but an understanding of what it means to hurt and what it means to be healed. Spirituality begins with the acceptance that our fractured being, our imperfection, simply is: There is no one to ‘blame’ for our errors — neither ourselves nor anyone nor anything else. Spirituality helps us first to see, and then to understand, and eventually to accept the imperfection that lies at the very core of our human be-ing. Spirituality accepts that ‘If a thing is worth doing, it is worth doing badly.’
Get the book here. 

The dismal fact is that self-respect has nothing to do with the approval of others — who are, after all, deceived easily enough; has nothing to do with reputation, which, as Rhett Butler told Scarlett O’Hara, is something people with courage can do without.
To do without self-respect, on the other hand, is to be an unwilling audience of one to an interminable documentary that deals with one’s failings, both real and imagined, with fresh footage spliced in for every screening. 
Character — the willingness to accept responsibility for one’s own life — is the source from which self-respect springs. Self-respect is a discipline, a habit of mind that can never be faked but can be developed, trained, coaxed forth. 
To have the sense of one’s intrinsic worth which constitutes self-respect is potentially to have everything: the ability to discriminate, to love and to remain indifferent. To lack it is to be locked within oneself, paradoxically incapable of either love or indifference.
Read more here. 

A characteristic of artistic education is for people to tell you that you’re a genius. […] So everybody gets this idea, if you go to art school, that you’re really a genius. Sadly, it isn’t true. Genius occurs very rarely. So the real embarrassing issue about failure is your own acknowledgement that you’re not a genius, that you’re not as good as you thought you were. […] There’s only one solution: You must embrace failure. You must admit what is. You must find out what you’re capable of doing, and what you’re not capable of doing. That is the only way to deal with the issue of success and failure because otherwise you simply would never subject yourself to the possibility that you’re not as good as you want to be, hope to be, or as others think you are.

I want your loves to be multiple. I don’t want you to be a snob about anything. Anything you love, you do it. It’s got to be with a great sense of fun. Writing is not a serious business. It’s a joy and a celebration. You should be having fun with it. Ignore the authors who say “Oh, my God, what word? Oh, Jesus Christ…”, you know. Now, to hell with that. It’s not work. If it’s work, stop and do something else.

Now, what I’m thinking of it, people always saying “Well, what do we do about a sudden blockage in your writing? What if you have a blockage and you don’t know what to do about it?” Well, it’s obvious you’re doing the wrong thing, don’t you? In the middle of writing something you go blank and your mind says: “No, that’s it”. Ok. You’re being warned, don’t you? Your subconscious is saying “I don’t like you anymore. You’re writing about things I don’t give a damn for”. You’re being political, or you’re being socially aware. You’re writing things that will benefit the world. To hell with that! I don’t write things to benefit the world. If it happens that they do, swell. I didn’t set out to do that. I set out to have a hell of a lot of fun.

I’ve never worked a day in my life. I’ve never worked a day in my life. The joy of writing has propelled me from day to day and year to year. I want you to envy me, my joy. Get out of here tonight and say: “Am I being joyful?” And if you’ve got a writer’s block, you can cure it this evening by stopping whatever you’re writing and doing something else. You picked the wrong subject.
See the speech here. 

‘Oh, dear me, how unspeakably funny and owlishly idiotic and grotesque was that ‘plagiarism’ farce! As if there was much of anything in any human utterance, oral or written, except plagiarism! The kernel, the soul — let us go further and say the substance, the bulk, the actual and valuable material of all human utterances — is plagiarism. For substantially all ideas are second-hand, consciously and unconsciously drawn from a million outside sources, and daily use by the garnerer with a pride and satisfaction born of the superstition that he originated them; whereas there is not a rag of originality about them anywhere except the little discoloration they get from his mental and moral calibre and his temperament, and which is revealed in characteristics of phrasing. When a great orator makes a great speech you are listening to ten centuries and ten thousand men — but we call it his speech, and really some exceedingly small portion of it is his. But not enough to signify. It is merely a Waterloo. It is Wellington’s battle, in some degree, and we call it his; but there are others that contributed. It takes a thousand men to invent a telegraph, or a steam engine, or a phonograph, or a telephone or any other important thing — and the last man gets the credit and we forget the others. He added his little mite — that is all he did. These object lessons should teach us that ninety-nine parts of all things that proceed from the intellect are plagiarisms, pure and simple; and the lesson ought to make us modest. But nothing can do that.’
Read more of Mark Twain’s letters here.