Logical Reasoning: Validity, Assumptions, and Evidence
Core concepts
Logical reasoning assesses argument analysis rather than memorized facts. Learn to identify premises, conclusions, and assumptions; distinguish deductive validity from inductive strength; and recognize common forms such as syllogisms, conditional statements, analogies, and cause-and-effect claims. For conditional reasoning, separate the sufficient condition from the necessary condition. A valid deduction follows necessarily from the premises; an inductive argument only makes the conclusion probable. Evaluate evidence for sample size, representativeness, and alternative explanations.
Exam traps
The most common error is reversing a conditional: from 'If it rains, the ground is wet,' you cannot conclude 'If the ground is wet, it rained.' Similarly, 'all A are B' does not imply 'all B are A.' Do not confuse correlation with causation, and do not accept an answer merely because it is true in the real world-it must follow from the passage. Watch scope shifts, such as 'some' becoming 'all,' and avoid extreme answer choices unless the evidence is equally extreme.
Worked example
Premises: All doctors are graduates. Some graduates work in laboratories. What follows? Nothing necessary about doctors working in laboratories; the overlap is possible but not guaranteed. The premises establish only that doctors belong to the graduate group, not that they share every property of some graduates.
Rule of thumb
Write the argument as premises → conclusion, then ask what must be true for the conclusion to follow.