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I notice there is already something cited there: ""However, as the document points out, the distinction between εκπορεύεσθαι (ekporeuesthai) and προείναι (proeinai) was not made in Latin theology, which used the same term, procedere, to denote both realities. Is this enough to explain the insistence of the Latin tradition on the Filioque? Saint Maximus the Confessor seems to think so. For him the Filioque was not heretical because its intention was to denote not the εκπορεύεσθαι (ekporeuesthai) but the προείναι (proeinai) of the Spirit."". But I still believe a more explicit citation should be found (or Maximus' view should be made more clear.   "