If a syllogism is valid, it’s premises will be arranged in such a way that only one conclusion is possible. This is the criteria that is important to logical validity. However, if the premises themselves are flawed, the conclusion may be factually inaccurate.For instance, consider the syllogism: “All dogs can fly. Fido is a dog. Fido can fly.” This syllogism is valid in terms of logic, but since the major premise is untrue, the conclusion is clearly inaccurate.
The structure of the argument made by a syllogism – the reasoning of the argument itself – is what you’re assessing when assessing a syllogism for logical validity.
 Observe the affirmative or negative nature of the premises and conclusion when looking to determine validity. Note that if either of the premises is negative, the conclusion must also be negative. If both premises are affirmative, the conclusion must also be affirmative. Further, at least one of a syllogism’s two premises must be affirmative, as no valid conclusion can follow two negative premises. If any of these rules are not followed, you already know the syllogism is invalid.


Further, at least one premise of a valid syllogism must contain a universal form. If both premises are particular, then no valid conclusion can follow. For example,“some cats are black" and "some black things are tables" are both particular propositions, so it cannot follow that "some cats are tables".
You’ll often simply know that a syllogism that breaks one of these rules is invalid without thinking about it, as it will likely sound illogical.
 Conditional syllogisms are hypothetical, and their conclusions are not always valid, as they depend on the condition that an unproven premise is true. Conditional syllogisms will include reasoning along the lines of “If_____, then_____.” These syllogisms are not valid when there are additional factors that may contribute to a conclusion.For instance: “If you keep eating Jolly Ranchers every day, you’re putting yourself at risk for diabetes. Sterling doesn’t eat Jolly Ranchers every day. Sterling is not at risk for diabetes.”
This syllogism is not valid for several reasons. Among them, Sterling may eat copious amounts of Jolly Ranchers several days a week – just not every day – which would still place him at risk for diabetes. Or, Sterling may eat cake every day, which would definitely place him at risk for diabetes.
 Syllogisms can allow for incorrect conclusions to be implied by a false argument. Consider the example: “Jesus walked on water. The green basilisk lizard walks on water. The green basilisk lizard is Jesus.” This conclusion is not necessarily true, as the middle term – in this case, “” – is not distributed in the conclusion.As another example: "All dogs love food" and "John loves food" does not logically indicate that "John is a dog." These are called fallacies of the undistributed middle, wherein a term that links the two phrases is never fully distributed.
Beware of the fallacy of the illicit major too. For instance, consider: "All cats are animals. No dogs are cats. No dogs are animals." This is invalid because the major term "animals" is undistributed in the major premise – not all animals are cats, but the conclusion relies on this insinuation.
The same may be said of an illicit minor. For instance: "All cats are mammals. All cats are animals. All animals are mammals." This is invalid because, again, not all animals are cats, and the conclusion relies on this invalid insinuation.
Distinguish between validity and “truth.” Even when a syllogism is logically valid, it may sometimes lead to a conclusion that is not factually true.
Look for linguistic giveaways that indicate invalidity.
Be skeptical of conditional syllogisms.
Watch out for syllogistic fallacies.
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