A key aspect of the Amsterdam Treaty receiving much attention in the IGC - not so much a leftover as a revisitation - is its main constitutional innovation: flexibility, or reinforced co-operation.
Amsterdam created a framework for co-operation on projects within the Union when not all member-states wish to join in. Previously, such arrangements were on an ad-hoc basis, as with the single currency or the social chapter, or outside the Union, such as the Schengen accord on passport-free travel.
But even before the ink was dry, the proponents of flexibility were worried its provisions were inoperable - and they have never been used. They complain the emergency brake provision for a veto allows individual states to block projects even if they are not going to be involved.
Attempts are also being made to reduce the minimum number of states which must be involved in such projects to seven or eight, instead of the present simple majority. The Spanish are proposing to extend flexibility to foreign and security policy, a prospect strongly opposed by Ireland.
Such reforms also have to be seen against a background of much talk of the need to create an avant garde, or a core group of member-states to provide dynamic leadership for the Union after enlargement. "I think differentiation, with a group of countries committed to forging ahead with closer co-operation in a clearly defined set of policy areas, will create a vanguard which will provide renewed impetus to the integration process," argues Mr Peter Sutherland.
It is this talk of a group, rather than a multiplicity of groups - Mr Joschka Fischler of Germany has spoken of the eurogroup as precisely such a vanguard - that causes alarm among the acceding states and some member-states. Is what is being conceived in reality a "two-tier" Europe with first and second-class forms of membership?
And what role will the Commission, the defender of the interests of small states, play?
Mr Brian Cowen has warned fellow ministers such changes may jeopardise the cohesiveness of the Union and has asked what specific projects the proponents of reform had in mind. An answer is not forthcoming, but Ireland will not want to be seen as blocking progress and will, at the end of the day, back the changes. The rationale is simple - any vanguard that does emerge will include us.
The Commission's new Secretary General, Mr David O'Sulivan, concedes it may be difficult to come up with credible projects for the use of reinforced co-operation but sees its importance as "a threat of last resort" that may never be used in practice. "My view is that in the end what we want is a mechanism that can be a threat to people who want to block particular decisions." The acceding countries remain to be convinced.