Let's talk about discrimination. People discriminate against one another for a myriad of reasons, but not all victims of discrimination are protected by the law. The Equality Act 2010 makes it unlawful to discriminate on the basis of one of seven protected characteristics: age, disability, gender reassignment, marriage and civil partnership, pregnancy and maternity, race, religion or belief, sex, and sexual orientation. If you face discrimination because of one of these characteristics - say, because you are disabled - you will normally have some form of legal recourse. However, if the discrimination is for an entirely different reason - perhaps you are being treated badly because you have red hair or are overweight - the Equality Act will not protect you.

Whether or not this is fair (and that's a debate for another day) it may, at first glance, at least seem straightforward. Discrimination is either because of a protected characteristic or it isn't... Right?

Not quite! There is an enormous amount of case law focused on what does and does not constitute a protected characteristic. In particular, the characteristic of "Belief" can be very difficult to pin down. The Equality Act says that "Belief" means "any religious or philosophical belief" (or lack thereof). But what is a philosophical belief? The Employment Appeals Tribunal (EAT) has considered this at length and a leading case found that, in order for a belief to be protected:

• The belief must be genuinely held (I can't just pretend I believe in something when I don't and expect protection).
• It must be a belief, not an opinion or viewpoint based on the present state of information available (I might hold the opinion that the Line of Duty season 6 finale was a bit of a let down, but this is not a belief that will garner me any legal protection).
• It must be a belief as to a weighty and substantial aspect of human life and behaviour (where someone comes down in the cats v dogs debate is unlikely to be deemed sufficiently weighty. For the avoidance of doubt, I am on team dog).
• It must attain a certain level of cogency, seriousness, cohesion and importance (if someone's actions in one instance are indicative of a certain belief, but their behaviour in other circumstances suggests something different, then the tribunal will probably find that the belief isn't that serious. I adore my parents' cat, so perhaps my commitment to team dog is not especially cohesive).
• It must be worthy of respect in a democratic society, not be incompatible with human dignity and not conflict with the fundamental rights of others (this one gets tricky and is often the key area for debate).

Last year, a tribunal found that veganism was capable of being a belief for the purposes of the Equality Act. However, this definitely does not mean that anyone with a passing interest in Veganuary will be afforded protection. The individual whom the judgement relates to practiced ethical veganism in every area of his life. He walks rather than taking a bus to avoid crashes with insects or birds and refuses to sit on leather seats; that sounds pretty cogent and serious to me.

Recently, the Equality and Human Rights Commission (EHRC) urged the EAT to grant the same level of protection to another belief: the gender critical belief that sex is biological and cannot be changed. This comes after the EAT heard an appeal from an individual who claims that her contract was not renewed because of her belief that trans women are not women. The tribunal in the first instance found that this was not a protected belief and the claimant has asked the EAT to revisit that finding; we are awaiting its judgment.

This is clearly an extremely controversial issue. It is not for the EAT to decide whether they agree with or support the claimant's views, but whether it falls within the definition of belief for the purposes of the Equality Act.

The tribunal who heard the original claim accepted that the claimant genuinely held the view that sex is biological and immutable and that, for her, this was more than an opinion based on available information - it was a fixed belief and she was not prepared to consider the possibility that it might be wrong. The tribunal accepted that the belief went to substantial aspects of human life and behaviour. The claimant believed that "a man is a person who, if everything is working, can produce sperm and a woman a person who, if everything is working, can produce eggs." While this view did not "sit easily" with her view that a person can only be male or female even if everything is not "working", and there were a number of minor inconsistencies in her views, the tribunal felt that the claimant's belief was generally cohesive.

However, the tribunal held that her belief was not worthy of respect in a democratic society and therefore failed the final test in the list above. It found that, because of the claimant's "absolutist" view of sex, she would refer to a person as what she deemed to be their gender, even if it violated their dignity or created an intimidating, hostile, degrading, humiliating or offensive environment.

The EHRC is concerned that, if the EAT upholds this ruling, it could leave people unprotected against discrimination and harassment simply because they hold controversial beliefs. Further (and this is where things get a little complicated), the EHRC notes that the fact that a belief is protected does not mean that comments made because of that belief are protected from consequences. The EHRC does not feel that the tribunal properly differentiated between the question of whether the belief was protected and the way it was expressed.

Indeed, that the manifestation of a protected belief is not itself protected is not a new idea. Past case law is clear that, while employers should not discriminate against employees for religious views or beliefs, this does not give employees free rein to express those beliefs in any way they like, regardless of the impact on others. For example, when a Christian registrar refused to perform civil partnerships due to her religious beliefs, the EAT found that she was not dismissed because of her beliefs (which were protected) but for her refusal to carry out her duties. The employer would have dismissed any registrar who refused to carry out civil partnerships, regardless of their religious beliefs. While this stance could have been a form of indirect discrimination, such indirect discrimination can be lawful when it is found to be (as it was in the case of the registrar) a proportionate means of achieving a legitimate aim.

So, if the EAT shares the EHRC's view that gender critical beliefs are protected, this does not mean that someone holding such a view could offend others with impunity. They would be protected from ill-treatment suffered as a result of their holding such a belief, but would not necessarily be protected from ill-treatment resulting from the way they behave because of that belief, for example by intentionally misgendering trans people. It's a fine line to tread and not always a straightforward distinction to make.

Additionally, gender-reassignment is itself a protected characteristic, meaning that - if the EAT agrees with the EHRC - employers and tribunals may face situations where they need to carefully weigh up the conflict between two different protected rights. To date, when the protected characteristic of religion and belief has clashed with another protected characteristic (say, sexual orientation), religion and belief has not usually come out on top.

In any event, this is a highly sensitive and important issue. The EHRC says that, while the protection offered by the Equality Act does "include highly controversial or offensive beliefs [it] does not include extreme beliefs such as a belief in racial superiority." There is certainly a case to argue that the belief that trans women are not women is extreme, and should not be deserving of protection. For many trans and gender-nonconforming people, a finding that these sorts of beliefs are protected would be completely devastating and directly at odds with their fundamental rights.