Kankhā: 'doubt', may be either an intellectual, critical doubt or an ethically and psychologically detrimental doubt. The latter may either be a persistent negative skepticism or wavering indecision. Only the detrimental doubt identical with vicikicchā is to be rejected as kammically disadvantageous, as it paralyses thinking and hinders the inner development of man. Reasoned, critical doubt in dubious matters is thereby not discouraged.
The 16 doubts enumerated in the Suttas e.g. M. 2 are the following: Have I been in the past? Or, have I not been in the past? What have I been in the past? How have I been in the past? From what state into what state did I change in the past? - Shall I be in the future? Or, shall I not be in the future? What shall I be in the future? How shall I be in the future? From what state into what state shall I change in the future? - Am I? Or, am I not? What am I? How am I? Whence has this being come? Whither will it go?
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.