# What is the difference between 'completeness' and 'soundness' in ZKP?

I'm reading Matthew Green's great blog, especially the Zero Knowledge Proofs: An illustrated primer post and I'm wondering what is the difference between completeness and soundness?

In the post, there is this part:

1. Completeness If Google is telling the truth, then they will eventually convince me (at least with high probability).
2. Soundness Google can only convince me if they're actually telling the truth.

Can I have some explanation? I don't understand the difference.

• A system where the verifier always accepts is complete. Google does (among other things) convince you if they're trying the truth. Is it sound though? – Maeher Jun 12 '18 at 23:07
• Completeness = "statement is true $\Rightarrow$ I become convinced" // Soundness = "I become convinced $\Rightarrow$ statement was true" – Mikero Jun 12 '18 at 23:58