A more thorough explanation:

The substantial-certainty test is a legal test used to determine whether a second work was copied from the first. The test asks whether a reasonable observer would conclude with substantial certainty that the second work is a copy of the first.

For example, if a songwriter creates a new song and another songwriter creates a song that sounds very similar, the substantial-certainty test would be used to determine if the second songwriter copied the first. If a reasonable observer would conclude with substantial certainty that the second song is a copy of the first, then the second songwriter could be found guilty of copyright infringement.

Another example could be if a company creates a new product and another company creates a product that looks and functions very similarly. The substantial-certainty test would be used to determine if the second company copied the first. If a reasonable observer would conclude with substantial certainty that the second product is a copy of the first, then the second company could be found guilty of patent infringement.

These examples illustrate how the substantial-certainty test is used to determine whether a work or product was copied from another. It is important to note that the test relies on the perspective of a reasonable observer, meaning that it is not based solely on the intentions of the person accused of copying.