Time is not a line with two equal directions: It is an arrow with different extremities. And it is this, rather than the speed of its passing, that matters most to us about time. This is the fundamental thing about time. The secret of time lies in this slippage that we feel on our pulse, viscerally, in the enigma of memory, in anxiety about the future.

Carlo Rovelli, The Order of Time (2019, Riverhead Books)