According to the usual definition, natural numbers do not include negative integers and zeros, so negative integers (such as-1, -2, -3) are not regarded as natural numbers. So, -7 is not a natural number.

The definition of natural numbers usually starts from 1, because they were first used by people to count the number of objects, such as an apple, two oranges, three birds and so on. Natural number is one of the basic concepts in mathematics. They have many important properties and uses, such as addition, subtraction, multiplication and division.

In order to understand the nature of natural numbers more deeply, we can consider the following points:

1. The difference between positive and negative integers:

Positive integers are integers greater than zero, including 1, 2, 3, 4, 5, and negative integers are integers less than zero, such as-1, -2, -3. Natural number set includes all positive integers, but excludes negative integers.

2. The nature of natural numbers:

Natural numbers have many important properties, including the following aspects:

Natural numbers are infinite, and there is no maximum natural number.

Each number in the set of natural numbers can be expressed as the previous number plus 1.

Natural numbers can be used for counting, arranging and measuring in mathematics and real life.

3. Symbolic representation of natural numbers:

Natural numbers are usually represented by n, N={ 1, 2, 3, 4, 5, ...}, where "…" means that the set of natural numbers is infinite.

To sum up, natural numbers are subsets of integers, including all positive integers, but excluding negative integers and zeros. Natural numbers play an important role in mathematics and are the basis of many mathematical concepts and theories. Therefore, it is very important to know and understand the nature and definition of natural numbers in mathematics and scientific research.