The weight - or any other facet - of a soul is something that a scientist is unlikely to ever seriously study. Researching such things could be career-limiting because the concept of a "soul" is fringe and decidedly non-scientific.
It can't be proven or disproven, which places it more in the domain of religion or philosophy.
However science is always probing around the edges and sometimes there's a person willing to give it a go.
In 1901, Dr Duncan MacDougall wanted to know if a soul has weight.
What became known as the "21 grammes experiment" deserves recognition in the list of odd experiments in the name of science.
It's hard to imagine how he arranged this, but he found six patients in nursing homes who were dying. He laid them on sensitive scales to measure whether their weight changed after their final breath.
Because he thought a soul would necessarily be very light, he wanted to ensure their movements minimised disturbance of the scales, so he chose people who were exhausted with conditions such as tuberculosis.
He tried to control weight loss due to evaporation and urination.
His answer? 21 grammes.
Unfortunately though, it didn't come remotely close to a reliable result.
Of the six, he discarded two because he hadn't been able to adjust the scales.
We get a sense of how people viewed what he was doing, when he wrote that he was having difficulties because of "... a good deal of interference by people opposed to our work..."
His measurements of the other four cases differed considerably, and he remarked that the experiment should be repeated many times to be validated.
Even then, the idea spread that a soul weighs 21 grammes.
He later tried similar experiments with 15 dogs, which he appears to have poisoned for the purpose. In a classic case of confirmation bias, he recorded no weight change: humans have a soul; a dog does not and therefore their weight will be unchanged.
Apparently, MacDougall's experiments on humans have never been repeated, probably with good reason. If rigorously conducted research was carried out - with ethics approval - it's not at all clear what it would actually prove.
If people were shown to lose weight after their final breath, does it follow that they have a soul?
A quirky, non scientific coda to this story is of Thomas Edison's final moments accompanied by his son, Charles. Charles noticed a rack of empty test tubes and, for some reason, decided to capture his father's final breath.
He sealed the tubes with paraffin and sent one to Henry Ford, where it's now on display in a museum.
Whether that contains a soul remains unproven.
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