|A Google-generated definition of pseudoscience, namely a belief, theory, or practice mistakenly regarded as being based on the scientific method.|
In 1919, at the age of 17, Karl Popper reflects on how there were many “theories” in the air, the three dominating his mind being: Albert Einstein’s relativity, Karl Marx’s theory of history (historical materialism), Sigmund Freud’s psycho-analysis (psychodynamics), and Alfred Adler’s individual psychology, and that the problem that troubled him was the following: 
“'When should a theory be ranked as scientific?' or 'Is there a criterion for the scientific character or status of a theory?' The problem which troubled me at the time was neither, 'When is a theory true?' nor, 'When is a theory acceptable?' My problem was different. I wished to distinguish between science and pseudo-science; knowing very well that science often errs, and that pseudo-science may happen to stumble on the truth.”
Popper states that he began to grapple with this problem in the “since the autumn of 1919”, after becoming impressed with recent May 1919 confirmation of Einstein’s light bending prediction of his relativity theory via Arthur Eddington’s solar eclipse expedition. 
|Left: a photo of Cambridge Observatory director Arthur Eddington who led an expedition to observe the total solar eclipse of May 29, 1919. During an eclipse, the sky gets dark enough that you can see stars, even close to the Sun. So Eddington set out to map the position of the stars when they were close to the Sun (right image), and see how the Sun bent the light (middle image) and to see if his measurements matched up with Einstein’s prediction, Newton’s prediction, or would light not bend at all? |
1. Popper, Karl. (1953). “Science: Conjectures and Refutations” (pdf), lecture at Peterhouse, Cambridge, Summer; originally published under the title “Philosophy of Science: a Personal Report”, British Philosophy in Mid-Century (editor: C.A. Mace). Publisher, 1957; in: Conjectures and Refutations: the Growth of (§1:43-77). Routledge, 2002.
2. Deflection of light by the Sun (section) – Wikipedia.
3. (a) Dowd, George L. (1929). “Eclipse to Check Einstein: Astronomers Journey Halfway Around the World to Study Five-Minute Spectacle, as the Moon Blots the Sun’s Face”, Popular Science Monthly (pg. 21), May.
(b) Siegel, Ethan. (2009). “The Last 100 Years: 1919, Einstein and Eddington”, ScienceBlogs, Jun 11.
4. Dear, Peter. (2001). “Science Studies as Epistemography”, in: The One Culture? A Conversation about Science (editors: Jay Labinger and Harry Collins) (§10:128-41). University of Chicago Press.
● Williams, Williams F. (2013). Encyclopedia of Pseudoscience. Routledge.
● Pseudoscience – Wikipedia.