There's long been a narrative among haters of Justice Clarence Thomas that he's not very intelligent and just goes along with whatever Justice Antonin Scalia does. The reverse is actually closer to the truth (but not all that close). It was Thomas' outside-the-box thinking that got Scalia to rethink a lot of the assumptions in his legal philosophy, and he was far more willing to take less moderate positions because of Thomas than he had been before Thomas was on the Court.
I've sometimes wondered if it's some kind of residual racism that's driving this narrative, with the stereotype of lower intelligence driving people to assume that Thomas is likely the less intelligent of the two, and since they so often vote together....
But no one ever suspected such a thing of Justice Thurgood Marshall, even though he so often voted with Justice William Brennan, the leader of the liberal wing of the Court for decades. So it's not just plain assumptions about black Supreme Court justices not being able to be as smart as white ones. More likely it's an assumption that no black justice who thought carefully and honestly would come up with the positions Thomas holds. Since I know people who explicitly hold such a view (when the reality is that no careful, intellectually honest, and fully-informed person could hold that view), this is highly plausible to me.
What's ironic, besides the fact that Thomas influenced Scalia more than the other way around and that Thomas is widely-viewed by Court-watchers across the political spectrum as one of the most original thinkers on the Supreme Court in decades, is that it turns out Marshall and Brennan may have in fact had the relationship that so many have accused Thomas and Scalia of having. According to a new biography of Justice Brennan by authors generally favorable to him, Brennan didn't think all that highly of Marshall as a justice. It's not that he was unimpressed at his intelligence. Anything but. He was so thoroughly impressed at his work as the chief counsel of the NAACP that he had high expectations of Marshall as a justice, and he simply failed to live up to them, except on a few issues, largely because (on Brennan's account) Marshall just didn't maintain the interest in the issues to think independently and carefully about them, pretty much just going along with whatever Brennan said in the way that many have claimed Thomas does with Scalia.
It was a complete surprise to me to read about this, because Marshall has long been heralded as a champion for liberal causes on the Supreme Court in ways that none have gone since he and Brennan left the Court. Most of the liberals on the current Supreme Court are noticeably closer to the mainstream on several issues, including capital punishment, affirmative action, and the intersection of first-amendment religion and speech rights. The idea that he chose not to think on his own and just went along with Brennan most of the time doesn't fit with the usual narrative.