Former Vice President Joe Biden Joe BidenThe Memo: Warning signs flash for Trump on debates Senate Republicans signal openness to working with Biden National postal mail handlers union endorses Biden MORE had a big night at Thursday’s Democratic presidential candidate debate in Houston. A lot was at stake, and I was not alone in focusing on the cogency of his answers.

Regarding Biden’s health care plans, former U.S. Housing and Urban Development Secretary and rival candidate Julián Castro accused him of confusing the details, "Are you forgetting what you said two minutes ago?"

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It was, of course, a pointed suggestion that Biden, at age 76, may not be as sharp as he once was. (His closest rivals in terms of polling, Sens. Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE of Massachusetts and Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE of Vermont, are 70 and 78, respectively).

Indeed, Biden appeared to struggle for cogency at times during the three-hours-long debate. At one point, he said parents of young children should "play the radio. Make sure the television — excuse me, make sure you have the record player on at night." Of course, records and record players haven't been in regular use for decades now.

I have never examined the former vice president, and so I cannot comment directly on his health. In fact, I was not concerned when I saw blood appear in his left eye during the CNN climate-change town hall last week.

I recognized that, since it didn’t appear to bother him, it was likely due to a benign, self-limited leak from a superficial vessel, known as sub-conjunctival hemorrhage. Though this problem often can be brought on by stress, high blood pressure or bleeding issues in an older person, and certainly looks striking on television, it is usually nothing to worry about, particularly if it doesn’t persist or recur.

What was more concerning to me, as a physician, was when Biden appeared to lose his train of thought mid-sentence.

In recent weeks, the former vice president has at times made what appeared to be cognitive errors. These include reportedly forgetting which state — Vermont or New Hampshire — he was in, when the Robert Kennedy and Martin Luther King assassinations occurred, and that he was not vice president when the Parkland, Fla., school shooting occurred.

Last weekend, at a campaign event in New Hampshire, Biden slipped and called the president “Donald Hump” before correcting himself and calling it a Freudian slip.

Biden had two brain aneurysms repaired surgically in 1988. The first one was fairly large, a centimeter in size, and was leaking. His neurosurgeon, Dr. Neal Kassell, has stated publicly that Biden is in excellent shape.

Still, the fact remains that studies have shown this type of aneurysm repair — especially when associated with a bleed — can sometimes lead to long-term cognitive difficulties, with problems in thinking, memory or orientation.

Of course, I cannot and will not say that such problems definitely apply to former Vice President Biden.

It is worth noting, however, that in the early 1990s a new technique was developed where brain aneurysms may be repaired directly through the artery (endovascular) and a coil inserted. This technique is generally safer and may well lead to fewer side effects in most cases.

Such questions about Biden’s health are not new for many presidential candidates.

In 2008, I was asked to fly to Phoenix to look through more than 1,000 pages of health records for Sen. John McCain John Sidney McCainKelly's lead widens to 10 points in Arizona Senate race: poll COVID response shows a way forward on private gun sale checks Trump pulls into must-win Arizona trailing in polls MORE (R-Ariz.), who was then the Republican presidential candidate.

Questions about McCain’s health arose because of concerns that he had a history of melanoma; the media was obsessed with the possibility of a recurrence, and I joined 22 journalists to pore through his files, like insects at a picnic. We found nothing of substance.

But I stated at the time that we had established a new standard: The days of hiding a president’s or a candidate’s mental and physical fitness to serve were over — and the public has a right to know.

Yet, that right does not include the right to speculate unfairly or without any basis in fact. Indeed, President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE has come under constant assault from the media and so-called medical experts who have never met or examined him, yet think it fair to question his mental fitness.

I am more interested in seeing actual medical records than in hearing speculation with a hidden agenda — and, in fact, two White House physicians have examined Trump and pronounced him to be fit.

Biden indicated Friday that he will release his health records before the Iowa caucuses. While I am glad to hear that, these records may not completely answer or address the questions his recent performances have raised.

I also believe we should see the health records of whoever emerges as the Democrats’ candidate for president in 2020, along with those of President Trump. If the Democratic candidate is Biden, given his prior history and recent concerns, I believe cognitive testing and a look at his neurological records would be reasonable.

But one thing that wouldn’t require testing is the man’s personal courage. Somehow, he soldiered on after the tragic traffic-accident death of his first wife and daughter in 1972, taking the oath of office as a newly elected U.S. senator in a ceremony at the hospital where his son, Beau, was recovering from severe injuries sustained in the same accident. And then, as vice president in 2015, he buried that same son after Beau Biden’s death from a brain tumor.

Joe Biden’s personal mettle clearly has been severely tested by life — and, when it comes to courage, he is more than fit to serve.

Marc Siegel M.D. is a professor of medicine and medical director at Doctor Radio at NYU Langone Health. He is a Fox News Medical Correspondent. Follow him on Twitter @drmarcsiegel.