"The lonely man finds himself surrounded by others with whom he cannot establish contact or to whose hostility he is exposed.
The solitary man, on the contrary, is alone and therefore "can be together with himself." In solitude, in other words, I am "by myself", together with my self, and therefore two-in-one, whereas in loneliness I am actually one, deserted by all others.
All thinking, strictly speaking, is done in solitude and is a dialogue of thought.
Solitude can became loneliness; this happens when all by myself I am deserted by my own self. Solitary men have always been in danger of loneliness, when they can no longer find the redeeming grace of companionship..."
To me this description in many ways parallels the differences between introversion and shyness. With introversion as an orientation of thinking more akin to solitude, and shyness more akin to loneliness. In that someone who is shy, may wish to be with others, but is afraid to reach out to them, and if ending up alone feels lonely; whereas an introvert being fully content with themselves, may selectively choose to be alone but without feeling lonely at all; with solitude being a positive experience of being alone, loneliness being a negative one.
I myself am a solitary person. Not a lonely person. I've never felt lonely before in my life. Not even as a little kid, when I was for awhile the new kid on the block who didn't have any friends. I've always felt perfectly at peace with myself, my greatest solace being nature and books. I've never felt completely alone because I've always felt a deep sense of companionship and connectedness with the earth and the sky and the greater universe.
If I were locked up in solitary confinement without a book or a window, that would be hell, but so long as I can be outside and see blue sky and stars and green grass and trees, and hear the sounds of nature and the sounds of birds singing, even by myself I wouldn't feel alone, wouldn't be lonely.
Of course that could all change in the blink of an eye, where solitude could easily digress into loneliness, where strength could be overtaken by weakness, where peace of mind could dissolve into misery...where if this connection to love is severed completely, all that would remain in its place is a feeling of separation, alienation, and isolation.
All it takes is a split second for your attitude to change completely, where your entire world could be turned upside down and inside out.
Attitude really is everything.
In order to never lose this feeling of connectedness, this "redeeming grace of companionship" with the world, it requires an attitude of openness and goodwill toward the whole world.
So that even in the absence of human companionship, you will never feel entirely alone, and even in the absence of housing or material possessions, you will never feel entirely homeless or deprived, but feel at home and among friends wherever you may be, even when alone you'll experience a peaceful solitude without loneliness.