title: Identity of blank nodes
body: <p>I am well aware of the way blank nodes are used, that they represent a locally-declared node with no IRI given. However, I am not sure how to interpret an actual logical &quot;meaning&quot; of a blank node. Does it have an identity? Is it different from any other blank node? Is it only an existential quantifier? What can be inferred from the corresponding knowledge base?</p>
  <p>Let's say we have this simple document:</p>
  <pre><code>&lt;iri1&gt; &lt;p&gt; [ &lt;p&gt; &lt;iri2&gt; ] .
  &lt;iri1&gt; &lt;p&gt; [ &lt;p&gt; &lt;iri2&gt; ] .
  </code></pre>
  <p>If the two blank nodes here only serve as existential quantifier, can it reduced to a single statement? Or does it mean there are (at least) two distinct entities, both in the same relation to <code>&lt;iri1&gt;</code> and <code>&lt;iri2&gt;</code>?</p>
  <p>What about this?</p>
  <pre><code>&lt;iri1&gt; &lt;p&gt; [ &lt;p&gt; &lt;iri2&gt; ] .
  &lt;iri1&gt; &lt;p&gt; &lt;iri3&gt; .
  &lt;iri3&gt; &lt;p&gt; &lt;iri2&gt; .
  </code></pre>
  <p>Is the first line here meaningful?</p>
  <hr />
  <p>My understanding is that unless the RDF document contains some sort of contradition when used with a logical framework (like OWL), then all the things <em>not</em> implied by the document can still be considered possible. This should also extend to <code>owl:sameAs</code>, and with that in mind it can be said that a blank node is just a regular node but with an &quot;IRI&quot; different from any other regular IRI.</p>
  <p>The problem with this interpretation is that it implies the opposite thing that I assumed at first – since <code>&lt;iri1&gt;</code> and <code>&lt;iri2&gt;</code> in the first document are not explicitly made distinct via <code>owl:differentFrom</code>, they &quot;could&quot; be one and the same thing (another document is free to link them). However, nothing can link the two blank nodes, and since people don't usually link everything with <code>owl:differentFrom</code>, we can know these blank nodes are distinct from each other (and every other node in existence), since they can never be unified.</p>
  <p>Which assumption is correct?</p>