These words all mean to set the mind to work in order to seek a better understanding of something, solve a problem or get at the truth. Think is the general word, and can refer to any use of the intellect to arrive at ideas or conclusions. One can think profoundly or superficially, seriously or frivolously: to think about whether man’s fate is determined by his own free will or by the force of circumstances; She thought about dyeing her hair red. Cogitate is a rather pompously formal word that means to think seriously or continuously; it is often used jestingly: The infant seemed to be cogitating on the quality and depth of his navel. Nevertheless it is occasionally used soberly about a baffling problem: tax specialists cogitating about how to simplify instructions to the taxpayer.

Meditate , muse , reflect and ruminate mean to think in a contemplative or leisurely manner. Meditate is the most general of these words, and implies a serious and extended period of concentration: The author meditated on the theme of his book before sketching out the plot and characterization. Muse suggests a dreamlike, aimless or conjectural succession of thoughts.

• She mused about whether her husband would notice her new dress: He mused over what he would do if he were suddenly to inherit a million dollars.

Reflect means to look back in a thoughtful way over what has happened: an old man reflecting on the changes that had taken place in the world since his youth. Reflect may be used also of any intellectual review: to reflect on the causes of student rioting. In its basic literal sense ruminate is applied to certain animals and means to chew the cud (food previously swallowed and regurgitate). Analogously, ruminate as here considered means to turn a thought over and over in the mind: a losing candidate ruminating on the cause of his defeat.

Deliberate and ponder emphasize the slow, careful process of weighing possibilities or alternative; deliberate stresses the slowness, and ponder the weighing, of possibilities. Whereas deliberate suggests a methodical, rational process of decision-making, ponder points to the solemnity and difficulty of the problem, to which there may be no solution.

• The jury deliberated four hours before bringing in a verdict; The Minister for Defence pondered the problem of how to maintain national security without compromising individual freedom.

To reason is to make logical or empirical generalizations based on evidence: The prosecutor reasoned that the suspect’s attempt to flee was grounds for presuming guilt. Speculate , on the other hand, means to theorize or conjecture on the basis of little or no evidence: At present scientists can only speculate on the nature and extent of life outside our solar system.