A comment that says only “I disagree” tells the writer where you ended up, but not how you got there. Reader critique becomes more useful when it identifies the passage, claim, or piece of evidence where the reasoning stops working for you.
Point to the Passage
Name the claim you are responding to, or describe the section closely enough that another reader can find it. Then explain what troubles you: a definition that shifts, an inference that seems too broad, or evidence that does not support the conclusion being drawn. Specificity makes disagreement inspectable.
Show Your Reasoning
A useful objection offers more than a verdict. Explain what you think the evidence supports instead, or what additional context would change your reading. If your response depends on an assumption, say what it is; that gives others a real point to consider rather than a position to guess at.
Leave the Question Open
Not every disagreement has to end in agreement, and a comment need not settle the essay's larger questions. It can still clarify what is at stake, reveal a gap, or suggest a better way to frame the issue. If you think the argument misses something, say where—and why.