These two symbols represent duality. They can be compared to the liǎngyí in Chinese science and philosophy. Their Chinese names would be yīn and yáng .

True and False:

The single line represents light. If it is light, things are visible. If something is visible, it exists. Therefore, if something exists, the condition is true.

The double line represents darkness. If it is dark, things are not visible. The absence of light means that something doesn’t exist. Therefore, if something doesn't exist, the condition is false.

Image Credit:

Duality: Lynn Fawcett, 2017.