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be under no illusion

To know and understand the reality or truth of a situation. Don't worry, I'm not getting back together with Alex. After all his lies, I'm under no illusion about how he operates.
See also: illusion, no

be under the illusion that

To be living, operating, or functioning based on a belief that is fanciful, unrealistic, or untrue. Jeremy's always been under the illusion that he's a great writer, even though he's never written more than a few crummy poems. No one likes paying taxes, but those who would call for them to be done away with altogether are under the illusion that our society can function without them.
See also: illusion, that

labor under the illusion of/that

To live, operate, or function with the unyielding belief in something, especially that which is fanciful, unrealistic, or untrue. Primarily heard in US. Jeremy's always labored under the illusion of being a great writer, even though he's never written more than a few crummy poems. No one likes paying taxes, but those who would call for them to be done away with altogether are laboring under the illusion that our society can function without them!
See also: illusion, labor, of, that
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

be under the illusion that

wrongly believe that.
1998 Independent The keening harmonies of the Brothers Gibb, a million naff dance routines by medallion men under the illusion that they were John Travolta.
See also: illusion, that

be under no illusion (or illusions)

be fully aware of the true state of affairs.
1992 Christian Scientist Monitor It is crucial to the nation's security…that we be under no illusions about reasons for this zero-loss rate.
See also: illusion, no
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
The optical illusion was created by researchers Oliver Flynn and Arthur Shapiro from the American University in Washington DC, reports BBC .
The gallery has rooms full of optical illusions spread over more than 400 square metres.
Unlike more 'classic' museums, visitors to Museum of Illusions Dubai are encouraged to interact with the exhibits and take as many photos as they like during their visit.
Summary: Museum of Illusions opens in Dubai in Al Seef featuring 80 interactive exhibits designed to play tricks on the brain
So, the researcher decided to bring models of language processing into the case and see if that provides an insight into the illusion. He worked with students at Kansas University (KU) and used an existing model known as the Node Structure Theory to find if the aspects of language processing covered in it explained the auditory illusion.
Wild Illusion, who won the Prix Marcel Boussac when last seen and also runs in the Godolphin blue, can be backed at 8-1.
IMPOSSIBLE features six of the world's most mind-blowing magical acts, fusing death-defying stunts, technological trickery, grand stage illusions and close-up magic in a fast-paced breath-taking performance that reinvents the biggest illusions in history.
A new study, published in Psychological Science, found that staring at a black and white spiral pattern rotating clockwise for 30 seconds gave participants the ability to see an average of 2.2 more letters on a logMAR eye chart than before looking at the optical illusion.
Thanks for checking out my Illusion. I created it and it still fools me.
The well-known rubber hand illusion is a prominent example of body schema manipulation (for the seminal study, see [7]).
Lady Gaga has released her new electrifying single entitled 'Perfect Illusion' on Thursday, her first solo since the release of her 'Artpop' album in 2013.
Somis, CA, July 11, 2016 --( Plan for an Afternoon or Evening of Illusion!
Themed 'Masters of Illusion,' the special programme will run from June 1-5 at the Katara Drama Theatre and explore the fascinating world of visual perceptions created by master filmmakers to provide audiences with a cinema experience that tricks the mind through the art of deception.