noun

• A cause:
• (uncountable) Rational thinking (or the capacity for it); the cognitive faculties, collectively, of conception, judgment, deduction and intuition.
  "Mankind should develop reason above all other virtues."
• (obsolete) Something reasonable, in accordance with thought; justice.
  "16th century Edmund Spenser, Lines on his Promised Pension I was promised, on a time, To have reason for my rhyme."
• (mathematics, obsolete) Ratio; proportion.
  "[I]f two Quantities repreſented by the Numbers 20 and 4 be compared, by dividing the Antecedent 20 by the Conſequent 4, the Quotient is 5; but inverting the Terms, by dividing 4 by 20 the Quotient is #92;tfrac#123;4#125;#123;20#125;#61;#92;tfrac#123;1#125;#123;5#125;. By which Quotients are declared the Geometrical Reaſons of the propoſed Quantities, becauſe if the Quotient found be multiplied by the Conſequent, the Product is equal to the Antecedent; for in the former Compariſon 5#92;times4#61;20, in the latter #92;tfrac#123;1#125;#123;5#125;#92;times20#61;4; as Things again are referred to Equality."

verb

• (intransitive) To deduce or come to a conclusion by being rational.
  ""I had," said he, "come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data. […]""
• (intransitive) To perform a process of deduction or of induction, in order to convince or to confute; to argue.
  "Still my spirit was not broken. I indulged the anticipation of escape, and that speedily. It was impossible, I reasoned, that men could be so unjust as to detain me as a slave, when the truth of my case was known."
• (intransitive, obsolete) To converse; to compare opinions.
• (ambitransitive) To arrange and present the reasons for or against; to examine or discuss by arguments; to debate or discuss.
  "I reasoned the matter with my friend."