I'm currently studying the textbook *Fundamentals of Electric Circuits*, 7th edition, by Charles Alexander and Matthew Sadiku. Chapter **1.7 Problem Solving** gives the following example:

> **Example 1.10**
>
> Solve for the current flowing through the \$8 \ \Omega\$ resistor in Fig. 1.19.
> [![enter image description here][1]][1]
> 
> **Solution:**
> 1. *Carefully **define** the problem*. This is only a simple example, but we can already see that we do not know the polarity on the 3-V source. We have the following options. We can ask the professor what the polarity should be. If we cannot ask, then we need to make a decision on what to do next. If we have time to work the problem both ways, we can solve for the current when the 3-V source is plus on top and then plus on the bottom. If we do not have the time to work it both ways, assume a polarity and then carefully document your decision. Let us assume that the professor tells us that the source is plus on the bottom as shown in Fig. 1.20.
> [![enter image description here][1]][1]

Notice that the authors have claimed (without justification) that the current is flowing *downwards* through the \$8 \ \Omega\$ resistor. In SpiRail's answer [here][2], they have also shown that, for the case where the polarity is + on the bottom, the current flows downwards through the \$8 \ \Omega\$ resistor (see the red arrow):
[![enter image description here][3]][3]
How does one know that the current is flowing *downwards* through the \$8 \ \Omega\$ resistor? This seems to be the immediate assumption, but there's no explanation from the authors as to why this is the case. 


  [1]: https://i.sstatic.net/tREDW.png
  [2]: https://electronics.stackexchange.com/a/596565/207355
  [3]: https://i.sstatic.net/TZYbC.png