> only messages should move between objects

Can you provide an example for this?

The Alan Kay viewpoint (he is NOT the inventor of OOP [1]) is considered the least helpful viewpoint on OO design. The “magical” and unhelpful “its all about messages” perspective, that helps you not at all unless one is talking about the internal implementation of a platform like Smalltalk. Consider the views of the real inventors - Nygaard and Dahl.

[1] I don't think I invented "Object-oriented" but more or less "noticed" what was really powerful about just making everything from complete computers communicating with non-command messages. This was all chronicled in the HOPL II chapter I wrote "The Early History of Smalltalk". — Alan Kay

Say you have a Car, Engine and Dashboard object.

Let's not have dashboard access the temperature by doing `GetSurroundingCar().engine.temperature`

If the dashboard needs to get the temperature from a sensor in the engine, it should be able to "talk" to the sensor, without going through car object.

In ideal OOP, a "method call o.m(...)" is considered a message m being sent to o.

In practice, field access, value and "data objects" etc are useful. OOP purism isn't necessarily helping if taken to the extreme.

The pure OOP idea emphasizes that the structure of a program (how things are composed) should be based on interactions between "units of behavior".

1. Avoid passing live instances (by reference) to other instances as much as possible. Because you don't want many instance references to be scattered too widely throughout your codebase. This can cause 'spooky action at a distance' where the instance state is being modified by interactions occurring in one part of the code and it unexpectedly breaks a different module which also has a reference to that same instance in a different part of the codebase. The more broadly scattered the reference is throughout the codebase, the harder it is to figure out which part of the code is responsible for the unexpected state change. These bugs are often very difficult to track down because stack traces tend to be misleading because they don't point you to the event which led to the unexpected state change which later caused the bug.

2. Avoid overly complex function parameters and return values. Stick to passing simple primitives; strings, numbers, flat objects with as few fields as necessary (by value, if possible). Otherwise, it increases the coupling of your module with dependent logic and is often a sign of low-cohesion. The relationship between cohesion and coupling tends to be inversely proportional. If you spend a lot of time thinking about cohesion of your modules (I.e. give each module a distinct, well-defined, non-overlapping purpose), the loosely-coupled function interfaces will tend to come to you naturally.

The metaphor I sometimes use to explain this is:

If you want to catch a taxi to go from point A to point B, do you bring a steering wheel and a jerry-can of petrol with you to give to the taxi driver? No, you just give them a message; information about the pick up location and destination. This is an easy to understand example. The original scenario involves improper overlapping responsibilities between you and the taxi service which add friction. Usually it's not so simple, the problem is not so familiar, and you really need to think it through.

We understand intuitively why it's a bad idea in this case because we understand very well the goal of the customer, the power dynamics (convenience of the customer has priority over that of the taxi driver), time constraints (customer may be in a hurry), the compatibility constraints (steering wheel and fuel will not suit all cars). When we don't understand a problem so well, an optimal solution can be difficult to come up with and we usually miss the optimal solution by a long shot.