I was a little doubtful about the propriety of going to the Mammoth Cave without a gentleman escort, but if two ladies travel alone they must have the courage of men. ↗
It may be doubtful, at first, whether a person is an enemy or friend. Meat, if not properly digested, becomes poison; But poison, if used rightly, may turn medicinal. ↗
Nothing seems to me more doubtful than Aristotle's remark that it is probable the arts and philosophy have several times been discovered and several times lost. ↗
Increasingly, stars are recruited from the ranks of professional models, with the result that today's starlets are better dressed and better groomed than ever before, though it is doubtful if they are better actresses. ↗