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Necessary Assumptions vs. Sufficient Assumptions

• Definition of an assumption: An unstated premise or condition that must be true for the conclusion to hold.
• Importance of assumptions in arguments: Assumptions form the backbone of logical reasoning and argument construction.
• Brief explanation of necessary and sufficient assumptions: These are two types of assumptions that serve different roles in arguments.

Necessary Assumptions – Definition

• Explanation of necessary assumptions: These are assumptions that must be true for the conclusion to be valid. Without them, the argument falls apart.
• Key features of necessary assumptions: They are essential for the argument but alone may not be enough to guarantee the conclusion.

Necessary Assumptions – Examples

• Argument 1: All dogs bark. My pet, Rover, is a dog.
  • Necessary Assumption: Rover is capable of barking.
• Argument 2: If it rains, the picnic will be canceled. The picnic was canceled.
  • Necessary Assumption: It rained.
• Argument 3: Every employee who works overtime gets a bonus. Jane got a bonus.
  • Necessary Assumption: Jane worked overtime.
• Argument 4: Vegetarians do not eat meat. Sam does not eat meat.
  • Necessary Assumption: Sam is a vegetarian.
• Argument 5: All citizens must pay taxes. John pays taxes.
  • Necessary Assumption: John is a citizen.

Necessary Assumptions – Discussion

• Recap of necessary assumptions: Necessary assumptions are crucial for the validity of the argument.
• Common scenarios where necessary assumptions are made: Everyday reasoning, hypothesis testing in research, legal arguments, etc.

Sufficient Assumptions – Definition

• Explanation of sufficient assumptions: These are conditions that, if met, guarantee the truth of the conclusion.
• Key features of sufficient assumptions: They are enough to ensure the conclusion but aren’t always necessary for the conclusion to be true.

Sufficient Assumptions – Examples

• Argument 1: John studied hard for the math test. Therefore, John will pass the math test.
  • Conclusion: John will pass the math test.
  • Sufficient Assumption: Studying hard for the math test guarantees passing the test.
• Argument 2: It snowed heavily last night. Therefore, school will be canceled today.
  • Conclusion: School will be canceled today.
  • Sufficient Assumption: Heavy snowfall the night before results in school cancellation.
• Argument 3: Sarah eats a balanced diet and exercises regularly. Therefore, Sarah is healthy.
  • Conclusion: Sarah is healthy.
  • Sufficient Assumption: Regular exercise and a balanced diet ensure good health.
• Argument 4: Our team won the final game of the season. Therefore, we will top the league.
  • Conclusion: We top the league.
  • Sufficient Assumption: Winning the final game of the season ensures topping the league.
• Argument 5: I can save $500 a month. Therefore, I can buy a car next year.
  • Conclusion: I can buy a car next year.
  • Sufficient Assumption: Saving $500 a month is enough to buy a car next year.

In each of these arguments, the premise leads to a conclusion. The sufficient assumption in each case is the unstated premise that the condition described in the premise leads directly and unerringly to the conclusion.

Sufficient Assumptions – Discussion

• Recap of sufficient assumptions: Sufficient assumptions provide enough conditions to ensure the conclusion.
• Common scenarios where sufficient assumptions are made: Planning, risk analysis, scientific predictions, etc.

Necessary vs. Sufficient Assumptions

Necessary assumptions are premises that must be true for the conclusion to be valid. If these assumptions are false or not met, the argument would fall apart.

Sufficient assumptions, on the other hand, if true, are enough to guarantee the conclusion, but they are not the only way the conclusion could be true.

Consider the examples:

Argument 1: John studied hard for the math test. Therefore, John will pass the math test.

• Sufficient Assumption: Studying hard for the math test guarantees passing the test.
• However, studying hard is not a necessary condition for passing. John might also pass if, for example, he is already highly proficient in math, he gets lucky in guessing some answers, or the test is easier than expected.

Argument 2: It snowed heavily last night. Therefore, school will be canceled today.

• Sufficient Assumption: Heavy snowfall the night before results in school cancellation.
• However, heavy snowfall is not a necessary condition for a school cancellation. School could be canceled for other reasons, like a power outage, a water main break, or other emergencies.

Argument 3: Sarah eats a balanced diet and exercises regularly. Therefore, Sarah is healthy.

• Sufficient Assumption: Regular exercise and a balanced diet ensure good health.
• However, a balanced diet and regular exercise aren’t necessary conditions for being healthy. Sarah could be healthy due to other factors such as genetics or perhaps she takes a medication that keeps her healthy.

Argument 4: Our team won the final game of the season. Therefore, we will top the league.

• Sufficient Assumption: Winning the final game of the season ensures topping the league.
• However, winning the final game is not a necessary condition for topping the league. The team could also top the league if the other leading teams lost their final games.

Argument 5: I can save $500 a month. Therefore, I can buy a car next year.

• Sufficient Assumption: Saving $500 a month is enough to buy a car next year.
• However, saving $500 a month isn’t a necessary condition for buying a car next year. The person could come into an inheritance, win the lottery, or earn a substantial raise at work.

These are just a few examples to illustrate why the assumptions listed are sufficient but not necessary. There are often multiple paths to a given outcome (conclusion), and a sufficient assumption represents just one of those paths.

Consider however the necessary assumption examples provided earlier: 

• Argument 1: All dogs bark. My pet, Rover, is a dog.
  • Necessary Assumption: Rover is capable of barking.
    This assumption is necessary but not sufficient. Just because Rover is capable of barking (assuming this to be true), it doesn’t guarantee that Rover will bark.
• Argument 2: If it rains, the picnic will be canceled. The picnic was canceled.
  • Necessary Assumption: It rained.
    The assumption that it rained is necessary but not sufficient. The picnic could have been canceled for a number of other reasons as well (e.g., a lack of attendees, food safety concerns, etc.).
• Argument 3: Every employee who works overtime gets a bonus. Jane got a bonus.
  • Necessary Assumption: Jane worked overtime.
    The assumption that Jane worked overtime is necessary but not sufficient. Jane might have received a bonus for reasons other than working overtime, such as excellent performance or reaching a sales target.
• Argument 4: Vegetarians do not eat meat. Sam does not eat meat.
  • Necessary Assumption: Sam is a vegetarian.
    This assumption is necessary but not sufficient. Sam might not eat meat for reasons other than being a vegetarian, like health issues or personal preference.
• Argument 5: All citizens must pay taxes. John pays taxes.
  • Necessary Assumption: John is a citizen.
    The assumption that John is a citizen is necessary but not sufficient. John might be paying taxes for reasons other than being a citizen, such as owning property or conducting business in the country.

In each of these cases, while the assumption is necessary for the conclusion to be valid, it is not sufficient to guarantee the conclusion because there could be other contributing factors or reasons.