Goudy on legibility:
Legibility depends on three things: first, simplicity, that is, a form having no unnecessary parts; second, contrast, as shown by marked differences in the weight of the lines composing the individual letters (stems and hairlines), and also shown in the varying widths of different letters; and third, proportion, each part of a letter having its proper value and relation to the other parts and to other letters—these three things in connection with the aspects of purpose and use.
Goudy on readability:
… a type without mannerisms, and that is easily and pleasantly readable, masculine, its forms distinct and not made to display the skill of the designer, but instead to help the reader. Type must be easy to read, graceful, but not weak; decorative, but not ornate; beautiful in itself and in composition; austere and formal, with no stale or uninteresting regularity in its irregular parts; simple in design, but not with the bastard simplicity of form which is mere crudity of outline; elegant, that is, gracious in line and fluid in form; and above all it must possess unmistakably the quality we call “arts”—that something which comes from the spirit the designer puts unconsciously into the body of his work.