Is there any difference in below two sentences?

X is supposed to be the reason of Y

X is believed to be the reason of Y

I feel that the first one (with "supposed") has a connotation of being a common belief that in fact can be wrong, i.e. in the first, one tries to imply that X is not the real reason of Y despite the widely held belief. The second one (with "believed") is just stating common belief. Is this the case? Or is there no real difference?

  • 2
    Side note: "...reason for..." is much more common than "...reason of...", although the latter is making a bit of a comeback. books.google.com/ngrams/…
    – Adam
    May 31, 2016 at 14:21

2 Answers 2


There are a variety of words in English that mean that people think an idea is true but it is not proven: believe, suppose, think, assume, presume, theorize, speculate, probably others. They indicate varying degrees of certainty, like "believe" would be more certain than "assume", but there is no simple ordering of them from least certain to most certain. There is definitely no quantification of certainty, like you can't say that "believe" means 72% certain while "think" means 58% or anything like that.

The level of certainty can be different depending on context and, in spoken English, tone of voice.

So "X is believed to be ..." and "X is supposed to be ..." essentially mean the same thing. Depending on context, one might indicate a higher level of certainty. There is no other difference.

Except, as politicallycorrect says, "supposed" can also mean "intended". This may or may not make sense in any given context. If someone said, "Radioactive decay is supposed to be the reason for the isotope distribution in this sample", I would take that to mean "is believed to be the reason". But if someone said, "Love is supposed to be the reason for marriage", they probably mean that love should be the reason.

  • Rather curiously, if you place stress on the word, as in X is believed to be true, it's actually quite likely you're going to continue with something that undermines that belief, rather than something emphasizing the strength of the belief. Presumably because you're drawing attention to the difference between believed and known. May 31, 2016 at 14:43
  • @FumbleFingers Quite true. "I BELIEVE Joe said he would be here at 9:00" is the sort of thing you say when you're not quite sure what Joe actually said -- or when the time has past and he still hasn't shown up. But someone reciting a creed, like "I believe in God the Father Almighty, maker of Heaven and Earth ..." normally says "believe" quite matter of factly and with no emphasis, and he is expressing a confidant assurance that this is so.
    – Jay
    May 31, 2016 at 17:20
  • My current coffee-table book is What We Believe But Cannot Prove, where I'm sure many people would place stress on both verbs to emphasize the contrast between belief and knowledge/proof. May 31, 2016 at 17:36
  • There's very little that you can prove with 100% certainty. Newton's Laws of Motion seemed pretty solid until Einstein came along and ruined everything. Everybody knew Troy was a myth until that upstart Schliemann found the place. Etc. Thomas Aquinas once said that one of the few things he was certain of was his own existence. Because, he said, I may be deceived about many things, but I cannot be deceived about my own existence, because in order to be deceived, I must exist!
    – Jay
    May 31, 2016 at 17:44
  • Tell that to the establishment / ruling classes / financial institutions in the UK, who are currently pulling out all the stops to "prove" that Brexit will inevitably lead to a financial crash, mass unemployment, World War III and genocide (and the Apocalypse, no doubt! :) May 31, 2016 at 18:00

Supposed implies that 'it is meant to be'. So X is meant to be the reason of Y, but it Isn't
The second, believed, is stating common beleif, you are correct. This implies that if X is the reason of Y, it is neither proven nor disproven

  • 1
    "Supposed" can mean "meant to be" or "intended", as you say. Like, "You are supposed to use a socket wrench for that bolt." But it can also mean "presumed" or "believed", As in, "It is supposed that this pyramid was built by the Maya." thefreedictionary.com/supposed
    – Jay
    May 31, 2016 at 13:19

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