“A conceptual level view of an object design describes the key abstractions. While someone might think of key abstractions as being nothing more or nothing less than high-level descriptions of "candidate classes", I prefer to consider a conceptual design from a slightly different angle--I'm thinking about design at a slightly different level.
An object-oriented application is a set of interacting objects. Each object is an implementation of one or more roles. A role supports a set of related (cohesive) responsibilities. A responsibility is an obligation to perform a task or know certain information. And objects don't work in isolation, they collaborate with others in a community to perform the overall responsibilities of the application. So a conceptual view, at least to start, is a distillation of the key object roles and their responsibilities (stated at a fairly high level). More than likely (unless you form classification hierarchies and use inheritance and composition techniques) many candidates you initially model will map directly to a single class in some inheritance hierarchy. But I like to open up possibilities by think first of roles and responsibilities, and then as a second step towards a specification-level view, mapping these candidates to classes and interfaces.”

About Conclusion
Designing scenarios: Making the case for a use case framework (1993)

Source: Object-oriented patterns. (1992), p. 152

Source: Object-oriented design: a responsibility-driven approach (1989), p. 71

Source: Object-oriented design: a responsibility-driven approach (1989), p. 75: Conclusion

The design of individual objects, and/or the design of the individual methods contained in those objects
The design of an inheritance (specialization) hierarchy of objects
The design of a library of reusable objects
The process of specifying and coding of an entire object-oriented application
The term nonformal is used to describe approaches to OOD that are not well defined, step-by-step, or repeatable, such as those that emphasize the design of individual objects, specialization (inheritance) hierarchies, and libraries of objects...
Abstract
Object‐Oriented Design (2002)

Source: Object-oriented design: With Applications, (1991), p. 37

Rob Pike (2004) comment in comp.os.plan9 http://groups.google.com/group/comp.os.plan9/msg/006fec195aeeff15 group at groups.google.com, 02-03-04

Source: Science and Sanity (1933), p. 20.
Context: The only link between the verbal and objective world is exclusively structural, necessitating the conclusion that the only content of all "knowledge" is structural. Now structure can be considered as a complex of relations, and ultimately as multi-dimensional order. From this point of view, all language can be considered as names for unspeakable entities on the objective level, be it things or feelings, or as names of relations. In fact... we find that an object represents an abstraction of a low order produced by our nervous system as the result of a sub-microscopic events acting as stimuli upon the nervous system.

Abstract
Designing scenarios: Making the case for a use case framework (1993)