A post or two below I had a crack at some words on 'Anglican validity' and a very interesting series of comments has ensued which has got me thinking about this word 'validity'. In connection with the eucharist of course, not my stamp collection. (Actually, the latter is non-existent).
I wonder if we use the same word in different ways and thus making some distinctions could be helpful.
Here are some ways I think may have been touched on in the comments below:
(1) Denominational validity: there are these rules pertaining to the ordering of eucharistic ministry. Sure they reflect great theological insights and powerful Scriptural injunctions and examples, but their sharp edges are provided by the denomination. Thus we Anglicans can entertain thoughts like 'a Methodist celebrating a Methodist eucharist in an Anglican church is offering a valid eucharist, but a Methodist (not having been ordained by a bishop) offering an Anglican eucharist in an Anglican church is offering an invalid eucharist.' The measurement of 'validity' at this point is 'our rules'. And, I do not think we are saying such a eucharist is theologically invalid (or, if you like, invalid from God's perspective). I suppose 'invalid' here has a strong sense of 'illegal.' Yet the rules are not rules for rules sake: the rules of denominations re eucharist reflect particular theologies of church, ministry, liturgy as well as eucharist.
(2) Transformational validity: there is a view that something happens to the bread and the wine when the eucharist is held, according to order, presided over by a correctly ordered person (so Anglicans, Lutherans, Roman Catholics, Old Catholics, Eastern Catholics, Eastern Orthodox and (perhaps depending who one is talking to) Presbyterians), and, depending precisely on the view held, those holding to the view accept a variety of denominational eucharists as valid, and a potential variety of correctly ordered persons presiding over the eucharist as being okay; and even where we do not think some things are correct we can respect the transformational intention of such churches. But the one thing many holding this view are united on is that a non-transformational intention (e.g. to steadfastly view the bread and the wine as nothing more or less than tokens or emblems) results in an invalid eucharist. Thus most Anglicans (so I understand) view Roman and Eastern Orthodox eucharists/masses/divine liturgy as valid eucharists (the exceptions could be certain evangelical Anglicans); and would see no need for a further ordination to take place of a Roman priest or E.Orthodox priest who (if we can imagine it) was willing to preside over an Anglican eucharist in an Anglican church. I understand Romans to view E. Orthodox divine liturgies as valid eucharists, but hold Anglican eucharists to be invalid ... yet I think they respect our eucharists in a way in which they do not (say) a Baptist or Brethren eucharist. It is on this sense of 'validity' that an Anglican might say that a Brethren or Baptist eucharist is invalid.
(3) Scriptural validity: all Christians undertaking eucharistic ministry aim to be faithful to the words of Scripture, both the narratival material concerning the last supper of Jesus, and the instructions of Jesus, reinforced in Paul's teaching. What Christians believe happens, or does not happen in respect of transformation of the bread and wine, and whether or not certain words are said as received by the church through tradition (such as the words known as the epiclesis or calling down of the Spirit), where Christians are faithful to Scripture in their enactment of eucharist, in this perspective, a valid eucharist is performed. Thus, as an Anglican, if I am a participant in a Baptist eucharist or a Brethren eucharist, I think I have been present at a valid eucharist. (I might also find it unsatisfactory in a number of respects and choose not to make it the form of eucharist I regularly attend).