Einstein was a physicist - He knows physics.
Newton was a physicist - He knows physics.
Lincoln was a politician - He knows political science.
Beethoven was a muscian - He knows music.
Plato was a philosopher - He knows philosphy.
If by "Law of Attraction," you mean how two opposites attract in terms of magnetic poles, then you may find proof in the many science textbooks around the world.
If by "Law of Attraction," you mean how two opposite people are attracted to each other, then that is something completely subjective and philosophical. In other words, it cannot be proven or disproven. Although you may find certain trends by studying statistics in psychology, these trends are just correlations. And as they say: Correlation does not imply causation.