“The mind is sharper and keener in seclusion and uninterrupted solitude. Originality thrives in seclusion free of outside influences beating upon us to cripple the creative mind. Be alone—that is the secret of invention: be alone, that is when ideas are born”
— Nikola Tesla

“When I am, as it were, completely myself, entirely alone, and of good cheer–say, traveling in a carriage or walking after a good meal or during the night when I cannot sleep–it is on such occasions that my ideas flow best and most abundantly”
— Wolfgang Amadeus Mozart

“On the other hand, although I have a regular work schedule, I take time to go for long walks on the beach so that I can listen to what is going on inside my head. If my work isn’t going well, I lie down in the middle of a workday and gaze at the ceiling while I listen and visualize what goes on in my imagination”
— Albert Einstein

“You need not leave your room. Remain sitting at your table and listen. You need not even listen, simply wait, just learn to become quiet, and still, and solitary. The world will freely offer itself to you to be unmasked. It has no choice; it will roll in ecstasy at your feet”
— Franz Kafka

“One can be instructed in society, one is inspired only in solitude”
— Johann Wolfgang von Goethe

“Without great solitude no serious work is possible”
— Pablo Picasso

I found these excellent articles by Leo Babauta a few days ago: The No.1 Habit of Highly Creative People and The lost art of solitude. They are an excellent dive into the value of solitude and they provide also some excellent quotes, like the ones above.