The statement that there is only one living substance capable of reproducing itself exactly is not entirely accurate.

Here's why:

* Viruses are not technically considered living organisms, but they do have the ability to replicate themselves inside host cells. While they don't have the same level of complexity as cells, they can create exact copies of their genetic material.

* Cells are the basic building blocks of life and can reproduce themselves through processes like mitosis and meiosis. This replication is not always 100% exact, as mutations can occur, but it's remarkably accurate.

So, while viruses and cells both have the ability to replicate, cells are the more complex and widely accepted definition of a "living substance".

It's important to note that even cells can sometimes make mistakes during replication, leading to mutations. These mutations are the driving force behind evolution.