WEDNESDAY afternoon and Sauchiehall Street is exquisitely bleak. The rain is nagging and cold. The dark – which has never really lifted since sunrise – is back with a vengeance. Head down, I hurry along the street. Ahead of me and to my right, I notice a man lying in a sodden sleeping bag on the pavement. He lies in a foetal position, his back turned away from the world that is rushing by. Just as I’m about to pass him, a woman appears from nowhere, swoops down over him and gently tucks a pack of sandwiches next to his clasped hands which are pressed up against his forehead, covering his face. The man doesn’t stir and carries on sleeping. The woman carries on walking. It happens so quickly and is so light-touch that there is something a bit dreamlike about it, almost as if it never really happened. A pre-Christmas story.

In witnessing this random act of kindness, I felt a slight twinge of something. Was it guilt because it hadn’t occurred to me to give the man anything other than my pity or shame? Then I wondered why the woman had done it, given that he was oblivious to her act of generosity. Was her sandwich donation an act of pure altruism? Or had she, or someone close to her, once been so on the edge that she could see through the rain-soaked heap on the ground to the person within?

This Tuesday (November 28) is "Giving Tuesday", a national campaign sponsored by the Charities Aid Foundation (CAF). It’s well-timed as a hangover cure for the over-indulgences of Black Friday. With support from the likes of Great British Bake Off winner Candice Brown, it aims to promote and help organise acts of kindness and giving to those in need. Participants are encouraged to bake, make and create in order to raise funds for charities of their choice. The proceeds will go to worthy causes in the UK and internationally. CAF also produces the World Giving Index, which measures the amount of generosity and altruism in 140 countries across the globe.

You could be forgiven for thinking that it’s the most developed countries whose people give the most in terms of their time, energy, compassion and cash. Not so. According to CAF’s last published Index for 2016, it was Myanmar whose people were the most generous. Iraqis were top of the compassion league when it came to helping strangers, and volunteering is most common in Turkmenistan.

It raises the question of why people really give to others, particularly when they have little enough for themselves. More intriguingly, how do we explain acts of extreme altruism such as running into a burning building to rescue someone we don’t know, donating a kidney to a total stranger or jumping into the sea to save a drowning dog? Darwinians and "selfish gene" theorists will tell you that there is no such thing as true altruism, that all acts are selfish and essentially biologically driven to ensure the survival of the species. But this doesn’t fully explain the woman who gave the sandwiches to the man in sleeping bag. It seemed to me that her giving was informed by a significant degree of moral intention, a kind of unconditional care that required no gratitude or acknowledgement.

Some would argue that there is always a selfish pay-off. In giving to others, we usually feel better about ourselves. The world, after all, seems to be made up of givers and takers. They both need each other. A world full of givers probably wouldn’t operate that well. A world full of takers would be woefully primitive. Maybe true altruism is rooted in a capacity for empathy and an awareness of our all too human vulnerability. There is also some science to back this up. The feeling centres of our brain, the amygdalae (almond shaped blobs of cells set deep in the temporal lobes), tend to be larger and more developed in those who are altruistic and highly empathic. Conversely, the amygdalae in psychopaths are noticeably smaller and less developed. There is an art to giving. At its most genuine, giving is a highly civilised, moral and political act. But when giving is compulsive, or indeed mindless; it’s unlikely to generate much in the way of good "karma".

When it comes to really good giving, it's the thought that counts.