This past weekend marked the swearing-in of Donald Trump as US president, and the moment in which the phrase ‘alternative facts’ joined ‘post-truth’ (the Oxford Dictionary’s most recent word of the year) and ‘fake news’ in our growing lexicon of Orwellian doublespeak. The occasion was the first clash of President Trump with the press, which had a bizarrely petty focus: the size of the crowds at his inauguration on Friday.

President Trump’s first speech at the CIA, on Saturday, attacked reporters and television networks for “lying” about the inauguration crowds and showing “an empty field” at the National Mall. “I looked out, the field was, it looked like a million, million and a half people,” Trump said. Later, press secretary Sean Spicer went on to defend Trump’s statement while chastising the media. “This was the largest audience to ever witness an inauguration — period — both in person and around the globe.” The proclamation was remarkable for its stark contradiction with verifiable data: birds-eye photographs showing considerably larger crowds at Barack Obama's first presidential inauguration in 2009. On Sunday, Counselor to the President Kellyanne Conway added fuel to the fire by insisting that Spicer had been truthful. “Sean Spicer gave ‘alternative facts,’” Conway said on NBC’s “Meet The Press.” The Merriam-Webster dictionary later weighed in on Twitter, reminding Conway that “A fact is a piece of information presented as having objective reality.” But, if Donald Trump’s electoral campaign and incipient presidency are any indication, the debate of what constitutes objective versus subjective reality is likely to endure.

We, the authors of this article, are neuroscientists specialized in the study of misperception and illusion. Our research focuses on the perceptual and cognitive errors that we make in everyday life, and on the clever tricks devised by painters and magicians to make us experience something other than what’s there. You could say that we study deception and misdirection for a living, two concepts that have become unexpectedly relevant to the political scene.

We have seen in the lab, over and over, that our senses are not to be trusted: no matter how assured we may feel in our perception of the events around us, we still may be dead wrong. A main part of the problem is that nobody experiences reality directly. Every sight, every sound, every feeling any of us has ever had has been filtered through the biological hardware and software of our brains—information processing machines made from tiny sacks of salt-water and protein. You have never experienced the world directly—only your brain’s simulation of it. This simulation may or may not match reality.

But even if our senses cannot fully grasp the world around us, there are precise rules to the game of obtaining unbiased knowledge, and ways of measuring objective reality. Here’s how the scientific method, and the science of illusion, can help:

Rule #1: We cannot ascertain what’s true, but we can establish what’s false.

Our picture of reality evolves every time we learn something new about the world. Sir Isaac Newton showed that Aristotelian physics was not the complete truth. In turn, relativistic and quantum physics superseded Newtonian physics. Each subsequent discovery has gotten us closer to the absolute, capitalized Truth about physics, but the possibility is always there that a new, unpredicted observation, will topple down accepted wisdom. Thus, a fundamental tenet of science is that, whereas no amount of data can verify a hypothesis, a single contradictory observation will refute it. In other words, hypotheses cannot be proven true, though they can be proven false. If there is one thing the scientific method excels at, it is disproving propositions.

Donald Trump’s hypothesis about the size of the crowd was possibly reasonable from his vantage point at the dais. As noted by the Washington Post, it may have looked to Trump as if the crowd stretched all the way to the back of the National Mall. Or maybe he just lied. Either way, hypotheses can only survive for as long as the data support them: from aerial photographs, to estimates from crowd scientists, to the public transit ridership numbers provided by the WMAT (Washington Metropolitan Area Transit), the facts reject the White House’s assertion that Trump’s inaugural crowd was the largest in history.

Rule #2: High confidence does not equal objective proof.

Remember the viral Dress? The social media phenomenon started with the photo of a dress, taken under ambiguous blue-and-yellow lighting. Roughly half of humanity saw the garment as white and gold; the other half saw it as blue and black. Both fractions felt equally confident in their assessment, and try as they might, they could not see the outfit in any other way. You might think of the two competing interpretations of the dress as two equally valid sets of ‘alternative facts.’ Except for this: if you illuminated the dress with a simple white light, then it would look blue and black to everybody.

We all can imagine alternate scenarios to any happening: sequences of events that might have occurred but never did. Conway might be excused if that was her meaning of ‘alternative facts.’ The recent Amazon Video TV series The Man in the High Castle is set in one such alternate reality, a dystopian universe in which the Axis powers won WWII. Perhaps there is an alternate reality in which President Trump had the largest audience ever to witness an inauguration. Nevertheless, it was not true in our universe.

The White House report about crowd size was not newsworthy due to its inaccuracy, but because of Trump’s (and later Spicer’s and Conway’s) high confidence in the incorrect information. Had Trump said that a million people seemed to be at the inauguration, but that he did not know the actual number, the account might have come across as more endearing—that the President was so moved by the event—than unsettling.

As an aside, the day after the inauguration we attended the Women’s March in Washington DC. Packed in like sardines, we had guessed that there must be several million people at the event, and were surprised later to find out that, based on overhead images, WMAT transit data, and crowd expert estimates, the march drew about ½ million people. The difference underscores how hard it is to assess the size of a crowd while being part of it—even for perceptual scientists accustomed to distrusting their senses.

Rule #3: Perception depends on perspective, but subjectivity is not a measure of reality.

Our neural wiring is such that it is virtually impossible for humans to think, or even see, in absolute terms. Our eyes do not count photons in the way a photographer’s light meter does. Rather, we see the world as a pattern of contrasts: the same grey circle might look black to us if surrounded by white, and white if surrounded by black. Our perception depends on context and perspective. We call illusions those cases in which our subjective relativism departs dramatically from the objective data (like when we see a grey circle as white, though the photographer’s light meter proves it’s not so). Some of the most dazzling misperceptions rely on the ingenious use of perspective. Kokichi Sugihara, a mathematician from Japan and repeat winner of the Best Illusion of the Year Contest, has built carboard ramps in which wooden balls seem to roll uphill. Yet, a different vantage point reveals that the uphill motion is just an illusion, and that the balls are rolling downhill in reality.

There is an ongoing discussion about how perception may have affected the White House’s claims about the size of the inaugural crowds. As mentioned above, there is the issue of Trump’s vantage point from the dais, which may have biased his perception towards larger numbers. This is not the White House’s conclusion. Instead, press Secretary Spicer took the tack of arguing that the aerial photos made Trump’s crowd look smaller than in real life, due to the unprecedented use of white floor coverings to protect the grass on the Mall. Whereas a crowd of people dressed in dark winter clothes should be generally easier to parse against a white background than against a dark one, contrast enhancement in the visual system will only take you so far. Not to mention the fact that white floor coverings were first used in 2013, for Obama’s second inauguration, and were still in place—but less visible due to the larger crowd—during the Women’s March, the day after the inauguration.

In our new era of fake news and post-truth gloom, the quest for objective truth and (non-alternative) facts has become more critical than ever before. Scientists and journalists must join forces in this common endeavor, and not hesitate to call out present and future falsehoods, whether due to innocent mistakes or to frank attempts to mislead. Whereas post-truth is an illusion—with no basis in reality—the actual truth is impervious to our wishes, emotions or beliefs. The scientific method teaches us that we will only ever attain truth by stubbornly stripping away every piece of misinformation that stands in its way. Investigative reporting and aggressive fact-checking will be crucial to get us there.