Ah. A misunderstanding.<p>What was meant was &quot;for each &lt;thing X that is done by complex software&gt;, &lt;a piece of software S&gt; doing X correctly is improbable enough that the existence of any piece of software doing X correctly may be considered to be impossible unless there is a piece of software that hasn&#x27;t been shown to do X incorrectly&quot;.<p>What was taken was &quot;for each &lt;thing X that is done by complex software&gt;, &lt;a piece of software S&gt; doing X correctly is improbable enough that the existence of any piece of software doing X correctly may be considered to be impossible unless there is a piece of software that hasn&#x27;t been shown to do &lt;any thing Y that is done by complex software&gt; incorrectly&quot;.<p>An important distinction.<p>Hopefully this makes things clearer.