Classic LISP semantics: In the LISP programming language, which atom denotes the logical value “false” and the empty list?

Introduction / Context:
LISP is foundational in AI research, with distinctive notions of truth, lists, and atoms. Understanding its truth values is important when reading classic AI code, rule systems, or symbolic processing utilities built in LISP or Scheme-like dialects.

Given Data / Assumptions:

• LISP represents both data and code as lists.
• The question asks which atom stands for “false.”
• We use the traditional Common Lisp semantics.

Concept / Approach:
In LISP, the atom nil denotes both logical false and the empty list. Any non-nil value counts as true in conditional contexts. The atom t is conventionally used to represent true explicitly, but truth is defined as “not nil,” so many non-nil values are truthy.

Step-by-Step Solution:
Recall LISP truth semantics: false == nil; true == any non-nil (often t).Map options: “t” is true; “nil” is false/empty list; “y”/“time” are unrelated atoms.Choose the correct atom for false: nil.

Verification / Alternative check:
Evaluating (if nil expr1 expr2) yields expr2; evaluating (if t expr1 expr2) yields expr1. The empty list prints as NIL in upper-case by convention in many LISP systems.

Why Other Options Are Wrong:
t: canonical true.

y, time: arbitrary symbols with no special truth semantics.

None of the above: incorrect because nil is the standard.

Common Pitfalls:
Assuming “false” and “empty” are separate values as in some languages; in LISP they are intentionally unified as NIL, simplifying certain list-processing idioms.

Final Answer:
nil