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Knowing Formal Logic is necessary for taking the LSAT. This content may be alien to you, so we start slowly and move through the topics in a simple and intuitive fashion.

The most basic logic concept is necessary vs. sufficient.

*Automatically **makes* something happens

** Qualification** for something

- 00:15 – The premises of a causal argument provide support for the hypothesis that one event causes another.
- 1:08 –
**Cause**is often used to describe something that has a specific outcome as a result. - 2:12 – A
**necessary cause**is an essential condition. It must be present for its corresponding conclusion to be present. - 2:28 – A
**sufficient cause**will always be enough to produce the effect. If it is present, then its corresponding conclusion must also be present. - 5:58 – There are
**four possible combinations**of necessary and sufficient conditions:**necessary but not sufficient**,**sufficient but not necessary**,**both sufficient and necessary**, and**neither sufficient nor necessary**. - 6:52 – If P is necessary for Q, then Q cannot be true unless P is true.
*Q is true only if P is true.*This can be diagrammed as follows:**Q → P**. - 7:36 – If P is sufficient for Q, then P’s being true is enough to make Q true.
*If P is true, then Q is true.*This can be diagrammed as follows:**P → Q**.

- 00:26 – To say that P is a necessary condition for Q is to say that “
*Q is true only if P is true*”. This can be diagrammed as follows:**Q → P**. - 00:44 – To say that R is a sufficient condition for S is to say that “
*If R is true, then S is true*”. This can be diagrammed as follows:**R → S**. - 1:01 – There are four possible combinations of necessity and sufficiency:
**necessary and sufficient**,**necessary but not sufficient**,**sufficient but not necessary**, and**neither necessary nor sufficient**.

Next LSAT: July 11th

Identify whether each of the following statements presents a **necessary condition**, a** sufficient condition**, **both**, or** neither.**

- A
**necessary but not sufficient**condition is essential for the conclusion to be true but not enough to produce the effect on its own. - A
**sufficient but not necessary**condition is enough to produce the effect on its own but is not essential for the conclusion to be true. - A condition that is
**both sufficient and necessary**is essential for a conclusion to be true and enough on its own to produce the conclusion. - A condition that is
**neither sufficient nor necessary**is neither essential for a conclusion to be true nor enough on its own to produce the conclusion.

(Don’t make any extraordinary assumptions when answering these questions).

Watering a plant is _______________ for it to grow.

**Necessary but not Sufficient**

Watering a plant is a necessary but not sufficient condition for it to grow. It is necessary because water is required for plants to grow. However, it is not sufficient because plants also need other things for growth, such as sunlight and proper soil.

Finishing college is _______________ for one to be rich.

**Neither Necessary nor Sufficient**

Finishing college is neither necessary nor sufficient for being rich. Some people did not finish college but still became rich. Moreover, finishing college does not guarantee that a person will become rich.

Knowing the correct formula is _______________ for finding the volume of an object in a math exam.

**Necessary but not Sufficient**

Knowing the correct formula is a necessary condition for solving the volume of an object in a math exam because using an incorrect formula will lead you to a wrong answer. However, it is not a sufficient condition because knowing the formula is not enough. You still have to do the calculation correctly to get the correct volume.

Given that the passing score for the midterm science exam is 70%, getting a score of 70% or higher in the exam is _______________ for passing it.

**Necessary and Sufficient **

It is both necessary and sufficient that one gets a score of 70% or higher in the midterm science exam in order to pass it. It is a necessary condition because the passing score for the midterm science exam is precisely 70%, so getting 70% or higher is required for you to pass it. Moreover, it is also a sufficient condition because getting a passing score automatically means that you pass the exam.

Living in Hong Kong is _______________ for living in Asia.

**Sufficient but not Necessary**

Living in Hong Kong is a sufficient but not necessary condition for living in Asia. If one lives in Hong Kong, then it automatically means that they also live in Asia, since Hong Kong is in Asia. However, it is not necessary because one can live in other countries in Asia other than Hong Kong.

If John usually leaves home at 8:30 am to not be late to his work which regularly starts at 9:30 am, given that his average daily commute time is 35 minutes, then it is _______________ that he leaves at least five minutes earlier so he won’t be late to his work tomorrow morning if it will start at 9:00 am for tomorrow’s shift.

**Both Necessary and Sufficient**

The first thing that we should take note of is John’s average daily commute time of 35 minutes. If he usually leaves home at 8:30 am, then he will arrive in his office at around 9:05 am. The fact that his work regularly starts at 9:30 am is irrelevant to this problem. Now, if his shift for tomorrow will start at 9:00 am then it is both necessary and sufficient that he leaves at least five minutes earlier so he won’t be late. It is sufficient because given that the commute will take 35 minutes, leaving five minutes earlier (8:25 am) will allow him to arrive at work on time (9:00 am). Similarly, this is also a necessary condition. One may argue that he does not have to leave five minutes earlier since he can leave 20 minutes or five hours earlier. However, the phrase used is “at least five minutes” which covers all times greater than five minutes. Hence, John will not be late to his work tomorrow if and only if he leaves earlier by five minutes or more than usual.

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Next LSAT: July 11th

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