think, cogitate, reflect, reason, speculate, deliberate mean to use one's powers of conception, judgment, or inference.

think is general and may apply to any mental activity, but used alone often suggests attainment of clear ideas or conclusions.

cogitate implies deep or intent thinking.

reflect suggests unhurried consideration of something recalled to the mind.

reason stresses consecutive logical thinking.

speculate implies reasoning about things theoretical or problematic.

deliberate suggests slow or careful reasoning before forming an opinion or reaching a conclusion or decision.

voluntary, intentional, deliberate, willing mean done or brought about of one's own will.

voluntary implies freedom and spontaneity of choice or action without external compulsion.

intentional stresses an awareness of an end to be achieved.

deliberate implies full consciousness of the nature of one's act and its consequences.

willing implies a readiness and eagerness to accede to or anticipate the wishes of another.