Definition of a link/connection

Let’s begin with something else. When we talk of ‘change’, it is by definition, change in the same thing - the same specific property of something.

When we compare 2 things, we can only compare their same corresponding properties. You cannot compare the weight of a book with the colour of a chair. So, when we say there is a similarity or a difference between 2 things, it is so between the same properties of the 2 things. 

The above leads us to the definition of a link - a typical connection between 2 entities : 

When 2 DIFFERENT properties of 2 entities are same/common, there is a link between the 2 entities. 

In fact, if there is a sameness between any 2 properties (one of each) of 2 entities, there is a link between the 2. In the second sentence, the definition is broadened to ANY 2 properties; that includes similarities as described in the first paragraph, as subsets of connections/links. 

If a chair is made of silver and a book has its name written on it in silver colour, there isn't any similarity between the 2 entities since different properties of the 2 entities bear sameness (material making up one and colour of ink of text written on the other) with each other, but a link.

The spotting of links between 2  entities is generally of value while thinking, in general.

An example of the above definition - Car and Petrol are connected - bear a link.

Firstly, what is the link? 

1. Car runs on petrol (car connecting to petrol)
2. Petrol resides in the tank of a car (Petrol connecting to car)

In the first case, the 2 properties (one of a car and one of petrol) - what car runs on, of a car - and what petrol is i.e. petrol itself, of petrol - bear sameness.

In the second case, the 2 properties (one of petrol and one of a car) - where petrol resides, of petrol - and what a car is i.e.car itself, of car - bear sameness.