Full Idea
Because he was attempting to classify abstract elements, not simple substances, Mendeleev was not misled by nonessential chemical properties.
Gist of Idea
Mendeleev focused on abstract elements, not simple substances, so he got to their essence
Source
report of Dmitri Mendeleev (The Principles of Chemistry [1870]) by Eric R. Scerri - The Periodic Table 04 'Making'
Book Reference
Scerri,Eric R.: 'The Periodic Table' [OUP 2007], p.118
A Reaction
I'm not fully clear about this, but I take it that Mendeleev stood back from the messy observations, and tried to see the underlying simpler principles. 'Simple substances' were ones that had not so far been decomposed.
Related Ideas
Idea 17399 Elements don't survive in compounds, but the 'substance' of the element does [Mendeleev]
Idea 17401 Mendeleev had a view of elements which allowed him to overlook some conflicting observations [Mendeleev]