The days blur into one. You have barely left the house for the last month, so when someone asks what you did yesterday, you draw a blank. You strain and strain and come up empty-handed. What distinguished yesterday from the day before it, or the day before that?

Much as lockdown has resulted in some people becoming inactive physically, a recent study suggests that our memories are weakening too. In certain conditions, it seems, people become mentally sedentary.

The idea that memory needs to be exercised is an ancient one. The Greek poet, Simonides of Ceos of the 5th Century BC, is said to have invented a system for training his memory when a dinner party ended in disaster.

He had popped outside to speak to a messenger when the roof collapsed, killing everyone apart from him. The collapse had made the bodies unrecognisable, but Simonides remembered their positions at the dinner table, and was able to identify everyone there.

From this grim discovery, he developed his method. By associating what you wanted to remember with places, and those places with images, you could store information away safely, strengthening your memory.

Soon afterwards, the art of mnemonics was born. Roman orators, such as Cicero drew on his system in order to memorise their famously long and complex speeches.

Some later scholars took such approaches to strange extremes. The 16th-century philosopher Giulio Camillo hoped to build a memory theatre that would allow those who stepped inside to instantly learn everything that could be known.

The process of associating memories with places, which has come to be known as the method of loci is still used today. Memory athletes often build "memory palaces" for this purpose.

Simonides' system, and other approaches, such as chunking, or the major system-where numbers are translated into consonants, then words-have helped them to perform feats such as memorising Pi to over 100,000 decimal places.

It is possible that these approaches to training the memory mirror how the brain itself functions.

Science now shows that a memory is not a single unit filed away in the cupboard of the brain. Rather, it emerges from a series of connections between neurons. A new network of connections is made between brain cells to create a new memory.

Simonides, Cicero and modern memory athletes enrich the associations of the things they want to remember. They are therefore developing the capacity to make connections.

But there are limits to memory athletics. The practices we have developed are good for holding onto specific knowledge, but sceptics point to the fact that memory athletes are not much better than ordinary people at remembering everyday occurrences. Not everything fits in the memory palace.

Yes it is, say some. It is clear that the brain can be trained to allow feats of memory that most people would consider impossible. Just as technology has reduced our need for physical exercise, it has reduced our need for memory. We may not need to know the periodic table when we can Google it, but that ability is available to those who are willing to do the work of reflecting and imagining.

Not so, say others. Marcel Proust famously described the experience of a memory he had been trying to recover in vain finally returning when he ate a Madeleine cake. Studies have since backed him up on the idea of an involuntary, situationally dependent aspect of our memory. We can improve some of our powers of memorisation, but memory itself is a more complex phenomenon.