Heres a few quotes on the subject by some of the greats.
" great photograph is a full expression of what one feels about what is being photographed in the deepest sense, and is, a true expression of what one feels about life in its entirety." - Ansel Adams
"An art is definable only in its own terms; it is as difficult to write about photography as it is about music, especially from a personal viewpoint. I feel that as one grows older his credo becomes simpler and more direct. Penetrating the smoke screens of equipment and techniques, glamor, ideology, and simple achievementmotive, the art of photography appears as strong and vital – and purposeful – as any other creative medium, and stands cleanly on its own feet. We are confronted today with a dichotomy; as our equipment and materials constantly grow in scope and quality the creative and technical standards appear to be diminishing; there is a near-cult of photographers who seem to intentionally avoid the beautiful and precise image, concentrating only on subject and obvious function. My personal reaction to this attitude is a determination to go as far in the opposite direction as possible. I believe in the most beautiful and appropriate prints, and the most clarifying and revealing approach of mind, heart, and craft. I believe that firm objectives in this directin can fulfill the promis of photography as one of the great visual arts. However, we must always be logical in our critical estimates; most of photography is not intended as art and should not be judged as such. But if art is intended, compromise must not be tolerated." - Ansel Adams
"You are asking me what makes a good picture. For me, it is the harmony between subject and form that leads each one of those elements to its maximum of expression and vigor." - Henri Cartier-Bresson
"To me, photography is the simultaneous recognition, in a fraction of a second, of the significance of an event, as well as of a precise organisation of forms which give that event its proper expression. I believe that through the act of living, the discovery of oneself is made concurrently with the discovery of the world around us which can mould us, but which can also be affected by us. A balance must be established between these two worlds- the one inside us and the one outside us. As the result of a constant reciprocal process, both these worlds come to form a single one. And it is this world that we must communicate. But this takes care only of the content of the picture. For me, content cannot be separated from form. By form, I mean the rigorous organisation of the interplay of surfaces, lines and values. It is in this organisation alone that our conceptions and emotions become concrete and communicable. In photography, visual organisation can stem only from a developed instinct." - Henri Cartier-Bresson
"Photography is, for me, a spontaneous impulse coming from an ever attentive eye which captures the moment and its eternity." - Henri Cartier-Bresson
"The difference between a good picture and a mediocre picture is a question of millimetres – small, small difference. But it’s essential. I don’t think there’s so much difference between photographers, but it’s that little difference that counts, maybe." - Henri Cartier-Bresson