This has been made especially obvious during the coronavirus pandemic: the virus is unseen as it transmits from person to person to person, inflicting suffering on many and asking some to pay the ultimate price.
What is unseen, when it is a virus, or an intruder at night, or the person whose footsteps we can hear behind us on a deserted street, can bring us great fear.
But there is beauty too in things unseen.
For a number of weeks now, a yellowhammer has been singing from a hedge at the front of our house.
A yellowhammer is a small yellow bunting, and because it nests in hedges late in the summer, it is vulnerable to hedge-cutting that takes place in late summer and early autumn, and because of this its numbers are tracked carefully by scientists and ornithologists.