If I understand
Jack's account of "Living Constitutionalism"
correctly, the basic idea is that (a) Justices will naturally pull the Constitution in the direction of what they see as sound public policy, and that (b) this is a good thing, as we end up with an updated Constitution over time that balances traditional Constitutional values (the rule of law, etc.) with modern needs. In other words, the system actually works pretty well without any fancy theory at all.
Assuming I'm correct about Jack's approach, here are two quick responses:
First, it is quite similar to that of my former boss, Justice Kennedy. As Justice Kennedy explained most clearly in his Academy of Achievement video interviews , his solution to the countermajoritarian difficulty is that the people want the Court to play a modest but important updating role. They want the Justices to use their insight to reinterpret the Constitution so that "every generation can invoke itsprinciples in their own search for greater freedom." The idea is basically the same as Jack's: The Court both enforces the Constitution against some majorities and yet also updates it slightly in response to changing majority views.
Second, I'm curious about how Jack would advise Justices tasked with deciding cases who need a bit more guidance than Jack's instruction to decide cases "the best way they can." Take the case of Justice Frankfurter, put on the Court by Roosevelt in 1939 back when judicial restraint was considered a liberal position. By the time Frankfurter left the Court in 1962, the Supreme Court's changing caseload had reversed the political valence of Frankfurter's model of the judicial role. Deference to the elected branches had become a conservative position rather than a liberal one. Would Jack have instructed Frankfurter that "times have changed," and that he needed to abandon his deferential approach? If so, why -- because Frankfurter was put on the bench to be a liberal, because new social movements were afoot, or both?