Rational
adjective
showing clear thought or reason:
He was too upset to be rational.
a rational course of action/argument/explanation

rationally
adverb
in a way based on reason rather than emotions:
Rationally, he knows that she won’t ever go back to him, but emotionally he can’t accept it.

1. believe- to think that something is true, correct or real:
Strangely, no one believed us when we told them we’d been visited by a creature from Mars.
[+ that] He believes that all children are born with equal intelligence.
[+ speech] She’s arriving tomorrow, I believe.
“Is she coming alone?” “We believe not/so (= We think she is not/is).”
[+ object + to infinitive] I believe her to be the finest violinist in the world.
[+ object + adjective] All the crew are missing, believed dead.

believable ]
adjective
If something is believable, it seems possible, real or true:
I didn’t find any of the characters in the film believable.

believer ]
noun [C]
a person who has a religious belief or has confidence in the benefit of something:
She’s been a believer since she survived a terrible car accident.
Harvey’s a great believer in health food.
I’m a great believer in allowing people to make their own mistakes.

presume (BELIEVE)
verb [T]
to believe something to be true because it is very likely, although you are not certain:
[+ (that)] I presume (that) they’re not coming, since they haven’t replied to the invitation.
[+ speech] You are Dr Smith, I presume?
“Are we walking to the hotel?” “I presume not/so.”
[+ object + adjective] The boat’s captain is missing, presumed dead (= it is believed that he is dead).
In British law, you are presumed innocent until you are proved guilty.
[+ object + to infinitive] The universe is presumed to contain many other planets with some form of life.

presumably
adverb
used to say what you think is the likely situation:
They can presumably afford to buy a bigger apartment.
Presumably he just forgot to send the letter.

presumption
noun [C or U]
when you believe that something is true without having any proof:
The presumption of innocence is central to British law.
There is no scientific evidence to support such presumptions.
[+ that] The decision is based on the presumption that all information must be freely available