In a 7-1 decision, the Supreme Court ruled in favor of Ferguson. The majority rejected Plessy’s 13th and 14th Amendment arguments, instead putting its stamp of approval on the doctrine of “separate but equal.” The dissent, written by Justice John Marshall Harlan, disagreed, arguing that segregationist laws indoctrinate society with the belief that the two races are not equal.

Justice Henry Brown wrote the majority opinion, which rejected Plessy’s argument that the Louisiana law conflicted with the 13th Amendment, deeming the point “too clear for argument.” The justices then considered whether the law conflicted with the 14th Amendment. They identified the purpose of the 14th Amendment as “enforce[ing] the absolute equality of the two races before the law,” but then asserted that “it could not have been intended to abolish distinctions based upon color, or to enforce social…equality.” According to the Court, the 14th Amendment was only concerned with legal, not social, equality.   

In addition, the justices denied the argument that separation of the races by law “stamps the colored race with a badge of inferiority.” They argued instead that racial prejudice could not be overcome by “an enforced commingling of the two races.” According to this argument, outlawing segregation would not eliminate racial prejudice, because such societal beliefs could not be changed simply by changing the law. The Court concluded that “if one race be inferior to the other socially, the Constitution … cannot put them upon the same plane.”    

The justices explained that because the Louisiana law did not conflict with the purpose of the 14th Amendment, the only remaining question was whether it was “reasonable, and … enacted in good faith for the promotion for the public good.” Giving much deference to the state legislature of Louisiana, they determined that the law met this requirement because it furthered “the preservation of the public peace and good order.” Thus, so long as separate facilities were actually qualitatively equal, the Constitution did not prohibit segregation in the view of the majority of the Court. 

Justice John Marshall Harlan dissented from the majority opinion. In an opinion that later became pivotal in the Brown v. Board of Education cases (1954), he argued that segregationist legislation, like the Louisiana law in this case, was based on the assumption that “colored citizens are so inferior and degraded that they cannot be allowed to sit in public coaches occupied by white citizens.” These laws promoted and perpetuated the belief that African Americans were inferior to whites, according to Justice Harlan. They must be struck down, he argued, because the government could not “permit the seeds of race hate to be planted under the sanction of law.” Justice Harlan believed that the constitution must be “color-blind,” and that it could allow “no superior, dominant ruling class of citizens.” Because segregation had the effect of creating such classes, he judged, it was unconstitutional.