His biggest contribution is to stress the ‘systems’ nature of the security and re

English as a Second Language English Vocabulary, Grammar and Idioms
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For many of those years, Dr. Neumann (pronounced NOY-man) has remained a voice in the wilderness, tirelessly pointing out that the computer industry has a penchant for repeating the mistakes of the past. He has long been one of the nation’s leading specialists in computer security, and early on he predicted that the security flaws that have accompanied the pell-mell explosion of the computer and Internet industries would have disastrous consequences.

“His biggest contribution is to stress the ‘systems’ nature of the security and reliability problems,” said Steven M. Bellovin, chief technology officer of the Federal Trade Commission. “That is, trouble occurs not because of one failure, but because of the way many different pieces interact.

I can’t sense the bold line. Could someone please explain it to me?

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I think
The security and reliability problems which occur are largely systemic by nature - they relate to a system rather than a whole part.
Dr Neumann was the one who pointed this out and made others aware of that fact.