Reductionism is a philosophy according to which the laws of physics are relatively simple and could be expressed concisely. If we were to understand and model all those laws we could explain any given phenomenon by breaking it down into its smaller parts until we just need to apply these simple laws.
With the current state of physics, it seems we are close to understanding many of these fundamental laws (although major shortcomings remain in the theory). Therefore, according to reductionism we should be close to understanding everything?
However as Murray Gell-Mann put it:
Reductionism is correct, but incomplete.
As a matter of fact, it was noted by Anderson (Anderson, n.d.) and many others (Gu et al., n.d.) that phenomena at different scales exhibit properties that couldn’t be reduced to lower scales, even though they are governed by the same exact laws. We need new models to describe these larger scale phenomena .
Gu, Mile, Christian Weedbrook, Alvaro Perales, and Michael A. Nielsen. n.d. “More Really Is Different” 238 (9-10):835–39. http://arxiv.org/abs/0809.0151.
Anderson, P. W.. n.d. “More Is Different” 177 (4047). American Association for the Advancement of Science:393–96. https://science.sciencemag.org/content/177/4047/393.