This passage comes from Tony Hoare’s contribution to Structured Programming, where he introduces abstraction as central to the design of reliable software. Here, however, he describes it as a general feature of human thought rather than a purely technical device.

Abstraction, as Hoare defines it, begins with noticing that different objects, situations or processes share certain features. One then chooses to focus on what they have in common and to set aside their differences, at least for the moment. This selective attention makes it possible to reason about many cases at once.

For Hoare, this was how the mind copes with complexity, in programming and beyond. By grouping like with like, a person can form concepts and structures that would be unmanageable if every detail had to be considered at once. The deliberate act of ignoring differences is what gives abstraction its power.