We have examined specific theoretical issues of computation over the years and have found our criteria for "properties of a problem solution" have evolved as our work has progressed. This changing perspective on computational techniques and solution models is reflective of the theoretical and technological advances we have observed. First we were satisfied if solutions to simple problems were computable and correct. Next we sought solutions that were elegant and generally applicable to an extended problem class. More recently, we have considered feasibility issues: finding problem solutions obtainable practicably, relative to real constraints of space and time.