Republican presidential candidate Ben Carson is defending himself against charges of hypocrisy after a doctor pointed out that Carson, a neurosurgeon, performed research in 1992 on tissue from an aborted fetus. Following the release of secretly recorded videos by an anti-abortion group, Carson has been condemning Planned Parenthood’s involvement in the donation of fetal tissue after abortions.

“To not use the tissue that is in the tissue bank regardless of where it comes from would be foolish. Why would anybody not do that?” Carson told reporters at a campaign event in Manchester, New Hampshire, on Thursday.

Carson told msnbc last week, and has since reiterated, that “it should be made very clear to people that the types of things we’re discovering by using fetal tissue can also be discovered by using non-fetal tissue. So it’s not like it is the only source as they try to make it sound.” But as physician Jen Gunter discovered and published on her blog Wednesday, Carson published a paper in 1992 that disclosed using tissue from ”two fetuses aborted in the ninth and 17th week of gestation.”

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“You have to look at the intent,” Carson told The Washington Post Thursday. “To willfully ignore evidence that you have for some ideological reason is wrong. If you’re killing babies and taking the tissue, that’s a very different thing than taking a dead specimen and keeping a record of it.”

While Carson seemed to be making a distinction between the people who perform the abortions, procuring fetal tissue, and those who perform research on the material, it’s not clear how the latter would exist without the former.

“There is absolutely no contradiction between the research I worked on in 1992 and my pro-life views,” Carson said in a statement provided to msnbc. “The issue of fetal tissue has everything to do with how the tissue is acquired. My primary responsibility in that research was operating on people to obtain diseased tissue for comparison to banked tissue samples. Killing babies and harvesting tissue for sale is very different than taking a dead specimen and keeping a record of it, which is exactly the source of the tissue used in our research.” Carson does not stand accused of performing abortions or processing the fetal remains for tissue.

Asked if the research could have been performed without fetal tissue, consistent with Carson’s claim that it is never medically needed, a campaign spokesperson responded in an email, “Specifically, that was 1992, this is 2015. Think there have been advances in medicine since then?”

Several medical experts disagree with Carson’s argument that fetal tissue isn’t crucial for research.

For example, at a legislative hearing on banning the practice in Wisconsin, Dr. Robert Golden, dean of the University of Wisconsin–Madison School of Medicine and Public Health and the university’s vice chancellor of medical affairs, explained that “There is incredibly important, potentially lifesaving research that goes on in Wisconsin that relies on fetal material received from federally regulated tissue banks.” Banning use of it, he added, “would have a substantial negative impact on our capacity to do the lifesaving research we are doing.”

Sheldon Miller, the scientific director of the intramural research program at the National Eye Institute, told The New York Times, “We couldn’t get this information any other way.”

Republicans have condemned Planned Parenthood for, they say, breaking the laws that regulate the legal donation of tissue after abortion, which the organization has denied. Some Republicans, including Rick Santorum and Rand Paul, have also condemned the entire practice, although when the ban on fetal tissue was debated in the early 1990s, many Republicans, including current Senate Majority Leader Mitch McConnell, voted to lift it.