Denialism is a person’s choice to deny certain particular facts. It is an essentially irrational belief where the person substitutes his or her personal opinion for established knowledge. Indeed, one of the hallmarks of denialism is a failure to recognise the distinction between opinions and facts.
Denialism should not be confused with modern scientific skepticism, which is the challenging of beliefs that are unscientific, irrational or based on insufficient evidence. Instead of denying facts, modern skeptics test claims by analysing whether they are supported by adequate empirical evidence. Denialism is the a priori rejection of ideas without objective consideration.
The philosophical skepticism of the Academic Skeptics and Pyrrhonists in Classical Greece (which was quite different to modern skepticism) consisted of doubting whether there can be any knowledge or facts at all, rather than denying particular facts.
Science denialism is the rejection of basic facts and concepts that are undisputed, well-supported parts of the scientific consensus on a subject, in favour of radical and controversial opinions of an unscientific nature. For example, the term climate change denialist is applied to people who argue against the scientific consensus that the global warming of planet Earth is a real and occurring event primarily caused by human activity.
The term evolution denialist or ‘creationist’ is applied to people who argue against the fact that life on Earth has evolved from earlier forms, instead of having been created by a supernatural being in its current form.
Other instances include Holocaust denialism, AIDS denialism, vaccination denialism, and Flat-Earthism. The various forms of denialism present the common feature of the person rejecting overwhelming evidence, often with attempts to deny the existence of a scientific consensus or alternatively to allege a conspiracy theory to fake or conceal the evidence. Denialism is commonly one of the foundations of quackery and other varieties of woo.
The motivations and causes of denialism include irrationality, religion and self-interest (political, economic or financial), beliefs in conspiracy theories or even defence mechanisms meant to protect the psyche of the denialist against mentally disturbing facts and ideas.